feat(sessions): Scrollable & collapsible queued-messages dock

[arthurdedeus]

Problem

While the agent is running, follow-up messages you submit get queued in a dock pinned above the composer. That dock had no height cap, scroll, or collapse — so with several long messages it grew unbounded, pushing the composer down, cropping the input at the bottom, and hiding the conversation above.

Closes #3150

Before

Changes

• Overflow — the queued-messages dock is now height-capped (max-h-[30vh]) and scrolls internally, so a long queue never pushes the composer off-screen. Added shrink-0 to the composer block in SessionView so the input is never compressed below its (now bounded) height; the flex-1 conversation thread yields the space instead.
• Collapse — a header row (N queued + caret) toggles the queue open/closed, backed by new per-task queueCollapsedByTaskId view-state in sessionViewStore (ephemeral, defaults to expanded, and survives the dock unmounting between turns). The live count stays visible while collapsed.
• Reuses the existing in-repo collapse pattern — @radix-ui/react-collapsible + phosphor carets, as in ProgressGroupView/SidebarSection — and the max-h … overflow-y-auto scroll idiom. The per-message Steer / Return to editor / Discard actions are unchanged.

After — expanded (scrolls instead of growing past the composer)

After — collapsed

Testing

• New QueuedMessagesDock.test.tsx — default-expanded shows every message with a count, the overflow classes are present, the header toggle collapses/expands the list (count persists), and an empty queue renders nothing.
• turbo typecheck, Biome lint, and the sessions component suite (100 tests) all pass.
• Manually verified in the running app (screenshots above).

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 9918a6a.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(sessions): Make the queued-messages..." | Re-trigger Greptile}