feat: soft-delete particles within a stream (#149)

Lets a particle's creator delete their own message from the TopBar
dropdown. Other viewers see a "This particle was deleted" tombstone in
place and playback auto-advances after ~2s, keeping indices stable for
concurrent watchers.

- Add optional deleted_at / deleted_by_human_id to non-container
  particle variants and isParticleDeleted helper.
- Add softDeleteParticle Firestore helper.
- New DeleteParticleOverlay confirmation and DeletedParticleView
  tombstone.
- Hide reactions (bar + 1-7 keybinding) on tombstoned particles.
- Show "Deleted particle" + Trash2 icon in the stream list preview.

Closes #146

https://claude.ai/code/session_01M2ShnZPvWfQzzvuu3Xm8b9

Co-authored-by: Claude <[email protected]>

js/src/lib/firestore-particles.ts

@@ -30,10 +30,12 @@ import type { Particle, ParticleType, ParticlePropertiesMap, Reactions } from "@
 const particleConverter: FirestoreDataConverter<Particle> = {
   toFirestore(particle: Particle): DocumentData {
     const { id: _id, created_at, updated_at, ...rest } = particle;
+    const deletedAt = "deleted_at" in particle ? particle.deleted_at : undefined;
     return {
       ...rest,
       created_at: Timestamp.fromDate(created_at),
       ...(updated_at && { updated_at: Timestamp.fromDate(updated_at) }),
+      ...(deletedAt && { deleted_at: Timestamp.fromDate(deletedAt) }),
     };
   },
   fromFirestore(
@@ -99,6 +101,8 @@ const particleConverter: FirestoreDataConverter<Particle> = {
           created_by_human_id: raw.created_by_human_id,
           updated_at: raw.updated_at ? (raw.updated_at as Timestamp).toDate() : undefined,
           reactions: raw.reactions ?? undefined,
+          deleted_at: raw.deleted_at ? (raw.deleted_at as Timestamp).toDate() : undefined,
+          deleted_by_human_id: raw.deleted_by_human_id ?? undefined,
         });
       }
       default:
@@ -364,6 +368,24 @@ export async function updateStreamStatus(
   await updateDoc(particleRef, { status, updated_at: serverTimestamp() });
 }
 
+/**
+ * Soft-delete (tombstone) a non-container particle. The Firestore doc stays
+ * in place so concurrent viewers see the deletion inline rather than being
+ * bumped to an adjacent particle. Idempotent — re-calling on an already
+ * tombstoned doc just refreshes the timestamp.
+ */
+export async function softDeleteParticle(
+  docPath: string,
+  humanId: string,
+): Promise<void> {
+  const particleRef = typedDoc(docPath);
+  await updateDoc(particleRef, {
+    deleted_at: serverTimestamp(),
+    deleted_by_human_id: humanId,
+    updated_at: serverTimestamp(),
+  });
+}
+
 export async function updateStreamPlaybackMarker(
   docPath: string,
   humanId: string,