mobile: task particles — view, edit, and compose (parity phase 3)

Bring the task particle to parity with desktop's richer model:

- Replace the thin `quest` schema with desktop's `task` model
  (ChecklistItem, TaskProperties: title/notes/checklist/assigned_to/done)
  in the discriminated union, the Firestore converter, and consumers
  (StreamCard, FallbackParticleView).
- New TaskParticleView renders an editable card (round done checkbox,
  title, notes, checklist with add/toggle/edit/remove, assignee chips)
  persisting each edit to Firestore; an 8s dwell auto-advances and
  field focus suspends playback. Wired into StreamView's render switch.
- Compose: a task button in the ComposeDock opens a TaskComposeSheet
  (createTaskParticle helper). Gated off in the new-stream flow, where a
  stream's first particle must be text or media.

Note: particles are written client-side to Firestore, matching desktop;
Orion's REST validator still only accepts `quest`, which is a pre-existing
inconsistency to reconcile backend-side separately.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01LqGPzXQ1AA9CqYHgCgmbtV

js/mobile/src/lib/firestore-particles.ts

@@ -97,7 +97,7 @@ const particleConverter: FirestoreDataConverter<Particle> = {
       case 'media':
       case 'file':
       case 'text':
-      case 'quest':
+      case 'task':
       case 'paper': {
         // Firestore stores timestamps as `Timestamp`; zod expects `Date`. Text
         // particles carry `properties.edited_at`, so coerce it if present.

js/mobile/src/lib/upload.ts

@@ -103,6 +103,32 @@ export async function createTextParticle({
   return createParticle(collectionPath, 'text', { content }, createdByHumanId);
 }
 
+interface CreateTaskParticleParams {
+  targetPath: ParticlePath;
+  title: string;
+  notes?: string;
+  createdByHumanId: string;
+}
+
+/**
+ * Create a `task` particle. Mirrors createTextParticle — the checklist and
+ * assignee are left empty and edited inline in the task card afterwards.
+ */
+export async function createTaskParticle({
+  targetPath,
+  title,
+  notes,
+  createdByHumanId,
+}: CreateTaskParticleParams): Promise<string> {
+  const collectionPath = toFirestoreChildrenPath(targetPath);
+  return createParticle(
+    collectionPath,
+    'task',
+    { title, done: false, ...(notes ? { notes } : {}) },
+    createdByHumanId,
+  );
+}
+
 function extensionFromMime(mime: string): string {
   if (mime === 'video/mp4') return '.mp4';
   if (mime === 'video/quicktime') return '.mov';