Add ad-hoc AI credit pack option to onboarding "Choose how to start"

[warp-agent-staging]

Summary

Free users who don't want a subscription had no way to get AI credits during onboarding. The Choose how to start offer (OfferVariant::ChooseHowToStart) only forked between subscribing and "Set up AI later", so anyone who just wanted to try Warp had to either commit to a plan or start with no AI at all.

This adds a third option that buys a one-time add-on credit pack.

Buy AI credits card. A new OfferChoice::BuyCredits sits between the subscribe and skip cards and presents the server's credit packs as a single horizontal row of tiles — credit count, the price actually charged, and the volume savings vs the smallest pack. Tiles are click-selectable and the three cards are arrow-key navigable. The layout follows the Billing & Usage page's add-on credit denominations row (compact credit chips 8px apart, with the same credits icon); the tiles are Expanded so the packs always stay on one line rather than wrapping.

Pricing is sourced at runtime, never hardcoded. current_credit_pack_options builds the list from pricingInfo.addonCreditsOptions and the viewer's PurchaseAddOnCreditsPolicy::effective_premium_bps, reusing price_usd_cents_with_premium so the displayed price matches what the server charges (free plan: $12 / $24 / $60 / $120 for the 400 / 1,000 / 3,000 / 6,500 packs). The option is hidden entirely when pricing hasn't loaded or the plan can't buy packs, and the packs refresh when pricing or the user's teams change.

Purchase and checkout return. "Get Warping" on that option calls purchaseAddonCredits with no teamUid, so the server resolves or creates a personal team for a brand-new free user. A PurchaseAddonCreditsCheckoutOutput opens the checkout URL and leaves the user on the slide; onboarding advances once a usage refresh shows the user has purchasable add-on credits. The balance is deliberately the purchased/add-on one rather than general AI availability (has_any_ai_remaining), which also counts base free-plan requests, BYOK, overages and auto-reload — so the case the flow exists for, a brand-new account with no credits that cancels checkout, stays on the slide. Someone who already holds credits and lands here can continue. A synchronous charge advances immediately; a rejection surfaces a retryable error.

Copy. The subscribe card is titled "Subscribe to a Warp plan". When the credit packs are on screen it adds the savings-framed line save 20% on add-on credits with any Build plan (matching the web wording, no "surcharge" language); when they aren't, it keeps its original copy.

Demo binary. crates/onboarding's standalone demo can now preview the post-auth offer slides via ONBOARDING_DEMO_OFFER=choose_how_to_start|head_start. It also had to start initializing feature flags — a debug build previously panicked on the first FeatureFlag::is_enabled check before any window rendered, so the demo was unusable for these slides.

Scoping note on ai_access_slide

Triage flagged whether "Get AI access" (ai_access_slide.rs) needs the same third option. It does not, and this PR deliberately leaves it alone: that slide is only constructed on the non-account-first path (agent_onboarding_view.rs builds ai_access_slide when AccountFirstOnboarding is disabled and offer_slide when it is enabled), so the two are alternative flows rather than forks that must stay in sync.

Rework changes

Round 1 — requester feedback on the first implementation.

• Card renamed. "Use Warp with AI" no longer distinguished the card once two of the three options were ways to use Warp with AI, so it is now "Subscribe to a Warp plan". The telemetry action name (use_warp_with_ai) is deliberately not renamed so existing dashboards keep continuity — called out in a code comment.
• Packs on one line. The pack selector previously stacked four full-width rows, which made the slide tall enough for the onboarding container to scroll. They now render as one horizontal row of equal-width tiles modelled on Billing & Usage. Each tile still shows all three data points; the only detail shed is the redundant "credits" word after each number, so CreditPackOption::credits_label() returns just the thousands-separated count ("6,500").

Round 2 — review finding on the advance predicate.

• The checkout-pending state advanced whenever has_any_ai_remaining() became true, which is broader than "the pack the user bought was granted", so a user with pre-existing AI availability could cancel checkout and still be advanced. The check moved to the purchased/add-on credit balance.

Round 3 — requester feedback on this PR (current).

• The add-on savings line is gated on the packs actually rendering. primary_description now takes the same shows_credit_packs condition that decides whether the buy-credits card renders, rather than a second predicate that could drift, and falls back to exactly the previous copy when the packs aren't shown.
• The intermediate purchase status strings are gone. "Starting your purchase…" and "Finish your purchase in the browser…" are deleted, not left dead; the "Waiting for checkout…" button label is now the only status the user sees. One deliberate exception: the failure line is kept, because it is terminal rather than transient — without it a rejected purchase would fail silently, with the button simply snapping back to "Get Warping". Happy to drop it too if you'd rather.
• All four pack tiles are the same height. The smallest pack has no volume discount and so rendered shorter. Every tile now lays out a badge slot; the no-discount tile lays out the same text fully transparent, which reserves exactly the right line box without a fixed-height constant.
• The advance rule is relaxed from "balance grew" to "balance is non-zero" (partially reverting round 2 per your note that it needn't monotonically increase). It stays scoped to purchased add-on credits rather than reverting to has_any_ai_remaining, so a brand-new zero-credit user who cancels still stays put while someone who already has credits can continue. canceled_checkout_does_not_advance_a_user_who_already_had_credits asserted the now-unwanted behavior and was replaced by canceled_checkout_does_not_advance_a_user_with_no_credits plus a_user_who_already_has_credits_can_continue.

Verification

Regression tests (new; the pricing math test is the guard on the confirmed server SKUs):

• app/src/pricing/pricing_tests.rs — 5 tests pinning the premium math ($10/$20/$50/$100 at list, $12/$24/$60/$120 at 2000 bps), volume savings 0/20/33/38%, empty input, and the no-negative-savings edge case.
• crates/onboarding/src/model_tests.rs — 11 tests over the purchase state machine, including the zero-credit cancel case, the already-has-credits case, and that completion callbacks are inert with no purchase in flight.
• crates/onboarding/src/slides/offer_slide_tests.rs — 7 tests over the slide: the third option appears only for the free-standard offer and only with packs, the subscribe copy with and without packs, arrow-key navigation, get_warping_buys_credits_when_the_credit_option_is_selected, "Set up AI later" still emitting while checkout is pending, and pack-list truncation.

Repository checks: ./script/format; cargo clippy -p onboarding --all-targets --tests -- -D warnings; cargo clippy -p warp --all-targets --tests -- -D warnings; cargo test -p onboarding (44 passed); cargo test -p warp --lib pricing:: (5 passed); cargo test -p warp --lib root_view (11 passed). All re-run after this rework and after merging the current master into this branch.

Visual verification (full-desktop screenshots and a recording below), run against the real UI via ONBOARDING_DEMO_OFFER=choose_how_to_start ./target/debug/onboarding:

• The three cards read "Subscribe to a Warp plan" (with the green Recommended pill), "Buy AI credits" and "Set up AI later".
• The four packs sit side by side on one line — 400 / $12, 1,000 / $24 Save 20%, 3,000 / $60 Save 33%, 6,500 / $120 Save 38% — and all four tile boxes are now the same height, with the 400 tile's blank badge slot bringing its bottom border level with the others.
• The entire slide — heading, all three cards and the bottom buttons — is visible at once with no scrollbar and nothing clipped.
• Starting a purchase leaves the user on the slide, the button becomes a disabled "Waiting for checkout…", and no status paragraph appears anywhere under or between the cards.
• Clicking the right-most pack moves the tile highlight; "Set up AI later" still advances.
• The no-packs fallback copy can't be exercised in the demo (it always supplies packs), so it is covered by subscribe_copy_drops_the_add_on_line_when_no_packs_are_shown instead.

Originating thread: https://warpdev.slack.com/archives/C0BDQDW8V5E/p1785523238898989

Computer-use screenshots

Full-desktop, uncropped captures with the app window surrounded by desktop, so the absence of clipping and of any scrollbar is visible rather than asserted. The demo runs under Xvfb with no window manager, so the window has no OS title bar and cannot be resized; the surrounding desktop and taskbar show its full extent.

Computer-use video recording

Continuous full-screen recording: default state, starting a purchase via the primary button, and selecting the last credit pack.

Conversation: https://staging.warp.dev/conversation/0bec429a-a2ca-46af-892b-4ce333e400ee
Run: https://oz.staging.warp.dev/runs/019fb990-9952-7194-b171-5995e49cc9de

This PR was generated with Oz.