feat(mobile): reorder and edit queued messages in place (port #3242)

[Gilbert09]

Problem

The mobile app lets you queue follow-up messages while the agent is mid-turn,
but you could only steer or discard them — there was no way to reorder the
queue or fix a message you'd already queued. This ports the desktop
capabilities (#3242) to mobile.

Changes

• Reorder queued messages with per-card move up/down controls; the order
  shown is the order they send.
• Edit in place: pull a queued message into the composer while it stays in
  the queue, then save it back into its original position (or cancel, leaving
  it unchanged). The composer shows an "Editing queued message" banner with a
  cancel action.
• Drain boundary: while a message is being edited it acts as a hard
  boundary — when the turn ends, only the messages queued before it
  auto-send; the edited message and everything after it stay queued until the
  edit is saved or cancelled.

Queue/edit logic lives in messageQueueStore (reorder, in-place update, and
the edit-hold that gates draining) and taskSessionStore
(flushQueuedMessagesIfIdle); the screen and dock are thin wiring. The
sendable-prefix boundary mirrors sendableQueuePrefixLength from
@posthog/shared, reimplemented locally because mobile's queue rows don't
match the shared AgentSession message shape.

How did you test this?

• Extended messageQueueStore.test.ts (reorder, edit-in-place update, edit
  hold set/clear + clear-on-remove, and the stopAtEdited drain boundary).
• Added flushQueuedMessagesIfIdle cases to taskSessionStore.test.ts
  (idle flush, edit-boundary flush, and no-op while running/terminal/compacting
  or empty). All 55 mobile store tests pass via vitest run.
• Ran biome check and tsc on the changed files — clean.

Automatic notifications

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

---

Created with PostHog Code

[trunk-io]

😎 Merged directly without going through the merge queue, as the queue was empty and the PR was up to date with the target branch - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 4db7ada.}