Phase 8: player view (local/projector) + sync seam

- Player View (/play): read-only projector screen — party HP bars + conditions,
  active-encounter initiative with current turn; enemy HP hidden behind a
  Healthy/Bloodied/Down status band; Fullscreen button
- SyncAdapter seam (src/lib/sync): local-first default via Dexie liveQuery;
  documents where a future networked (WebSocket/CRDT) backend plugs in
- Dashboard + routes get a Player View entry
- Robustness: encountersRepo.mutate() does transactional read-modify-write so
  rapid combat mutations can't overwrite each other (old C19 race); tracker uses it

New player-view e2e. Networked multiplayer backend intentionally deferred.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/db/repositories.ts

@@ -164,6 +164,19 @@ export const encountersRepo = {
     await db.encounters.put(validated);
   },
 
+  /**
+   * Apply a transition by reading the freshest committed state inside a
+   * transaction, so two rapid mutations can't overwrite each other (the
+   * read-modify-write race the old app shipped — bug class C19).
+   */
+  async mutate(id: string, fn: (e: Encounter) => Encounter): Promise<void> {
+    await db.transaction('rw', db.encounters, async () => {
+      const current = await db.encounters.get(id);
+      if (!current) return;
+      await db.encounters.put(encounterSchema.parse({ ...fn(current), updatedAt: now() }));
+    });
+  },
+
   async remove(id: string): Promise<void> {
     await db.encounters.delete(id);
   },

src/lib/sync/index.ts

@@ -0,0 +1,26 @@
+/**
+ * Sync seam.
+ *
+ * Today the app is local-first: all data lives in Dexie and the "player view" is
+ * driven on the same device by liveQuery, so there is nothing to sync. This
+ * interface is the seam a future networked adapter (e.g. a WebSocket/CRDT
+ * service) implements to mirror repository writes to other devices — without the
+ * feature code needing to change.
+ */
+export type SyncStatus = 'local' | 'connecting' | 'connected' | 'error';
+
+export interface SyncAdapter {
+  readonly status: SyncStatus;
+  /** begin syncing this campaign (no-op locally) */
+  start(campaignId: string): Promise<void>;
+  stop(): void;
+}
+
+/** Default adapter: single-device, backed entirely by Dexie liveQuery. */
+export const localSync: SyncAdapter = {
+  status: 'local',
+  async start() {
+    /* nothing to do — same-device sharing is automatic via liveQuery */
+  },
+  stop() {},
+};