feat(updates): add setting to dismiss update banners

[charlesvien]

Problem

#3101 made the update-ready banner dismissible by adding a "Later" button, which grew the banner into a two-row card. Update banners are meant to stay concise, and dismissal should be opt-in without changing the default UI.

Changes

• Revert feat(updates): add "Later" option to dismiss the update-ready banner #3101, restoring the original single-row banner layout.
• Add a "Dismissible update banners" switch to Settings > Updates (persisted, default off, only shown when updates are enabled).
• With the setting on, hovering a banner reveals a small dismiss X: an overlay badge on the corner of the sidebar cards and an inline icon next to the compact title-bar pills. The banner layout itself never changes, and with the setting off nothing renders differently from before the revert.
• Dismissal is session-scoped and keyed by version: one click hides every banner instance for that version and keeps it hidden through the available/downloading/ready cycle. The banner returns when a newer version arrives, after an app restart or when the setting is turned off.

Screenshots

At rest (setting on)	Hovered

Screenshots come from the dev app with a forced update state, since the updater is disabled in dev builds. Images are hosted on the pr-3107-assets branch; delete it after merge if unwanted.

How did you test this?

• UpdateBanner.test.tsx (7 cases): no dismiss button when the setting is off and dismissal when it is on, both parameterised across the sidebar and compact variants; dismissal holds across the update cycle; a newer version resurfaces the banner; turning the setting off restores a dismissed banner.
• Added dismissibleUpdateBanners to the settings store rehydrate test table.
• pnpm typecheck (22 packages), biome check on the changed files and the full @posthog/ui vitest suite all pass.
• Drove the real Electron app over CDP with a forced update state to verify the hover reveal live and capture the screenshots above.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 601f278.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "add setting to dismiss update banners" | Re-trigger Greptile}

[stamphog]

T1d feature with zero reviews that silently removes the always-visible "Later" dismiss button from the update-ready banner for all existing users — it's now gated behind a new setting that defaults to off. This is a user-facing behavioral regression that warrants human sign-off before merging.

[stamphog]

The PR removes the always-visible "Later" dismiss button from the update-ready banner and replaces it with a new dismiss button gated behind a setting that defaults to off — a user-facing behavioral regression for all existing users. A bot review flagged this exact concern on an older commit, and the current diff still exhibits the same behavior. No human has reviewed this change.