[Bug]: Mobile new-task draft is saved under Unsent but not resumed when navigating away #15974

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/mobile

Steps to reproduce

1. On Android, tap + to start a new task (NewTaskSheet -> NewTask composer).
2. Type prompt text but do not send.
3. Press Android back / navigate away (dismisses the sheet via closeNewTask -> parentNavigation.goBack()).
4. Tap + again to start a new task.
5. Check the thread list Unsent section.

Expected behavior

Returning resumes the same draft in the composer (like web, which reuses/resumes the per-project draft via stable DraftId). At minimum, navigating away should flush the pending draft write so nothing is lost.

Actual behavior

Update 2026-10-05: field screenshot confirms the typed text IS saved — it appears under the thread list Unsent section as a Draft row (e.g. "sync with upstream. on mobile"). So the original "not saved as draft" report was imprecise.

The remaining gap is resume, not persistence: the composer binding (activeDraftKey provider-local useState) is lost on sheet unmount, and re-entry always mints a fresh draft ("A fresh New Task never reuses one"), so the composer opens empty and the user must find and reopen the Unsent draft row manually. Secondary edge: dirty text does not guard back navigation, and there is no flushComposerDrafts() on sheet dismiss, so typing + backing out within the 200ms persist debounce can still lose the disk write.

Code pointers (main @ cf3e714b0f):

• apps/mobile/src/features/threads/new-task-flow-provider.tsx:291 (activeDraftKey provider-local state, lost on unmount), :466-476 (fresh draft mint), :589 (never reuses), :651-658 (setPrompt -> setComposerDraftText), :753-763 (in-flow retarget is fine)
• apps/mobile/src/features/threads/NewTaskDraftScreen.tsx:1412-1459 (composer), :514-518 (usePreventRemove only while submitting/transferring, not dirty text), :1463-1471 (close/goBack)
• apps/mobile/src/state/use-composer-drafts.ts:66,1000-1025 (200ms debounced persist), :777-807 (flushComposerDrafts never called on dismiss/unmount)
• Web contrast: apps/web/src/composerDraftStore.ts:89,97-115,132-137 (localStorage + debounce + beforeunload flush), threadRoutes.ts:83 + hooks/useHandleNewThread.ts:164-195 (stable DraftId resume)

Impact

Minor bug or occasional failure

Version or commit

main @ cf3e714

Environment

Android (mobile app), reported from field testing

Logs or stack traces

None - text is preserved under Unsent but not resumed; no error.

Workaround

Reopen the saved draft via its Unsent Draft row (usePendingTaskListActions.ts:16-30 -> NewTaskDraft -> { draftId }).