feat(persistence): local-first story persistence (IndexedDB + Supabase skeleton)

Remove Cloudflare D1 entirely (4 API routes, lib/db/, Drizzle config/migrations,
drizzle-orm/drizzle-kit deps, wrangler D1/R2/KV bindings) and replace with
browser-local-first architecture:

Open-source build (IndexedDB, no auth):
- lib/persistence/ 5-file module: types, idb adapter (zero-dep, fault-tolerant,
  post-open invalidation retry), localStore (CRUD + sync-reserved metadata +
  slim/rebuild + retention-cap eviction with tombstone reap + sync-state
  protection + last-resort bounded fallback), sessionSlim (voice strip +
  styleRef absent-delete), cloudStore (Supabase skeleton, server-only)
- Autosave: persistence fingerprint (history.length:lastBeatCount:playerName),
  serial saveChain, failure rollback retry, replaySourceRef guard (prevents
  replayed shared stories from clobbering user saves)
- clientStoryPersistence.ts: thin facade (SaveResult discriminated union)
- Stories page: /[locale]/stories with 3-language i18n (zh-CN/en/ja)
- Homepage: book icon entry point in header

Commercial build (Supabase, skeleton only):
- Single table public.stories (JSONB + RLS 4 policies on auth.uid()=user_id)
- supabase/migrations/ DDL (idempotent)
- cloudStore.ts server-only repository, AUTH_ENABLED short-circuit
- Not wired to client this phase

Featured stories: pure fallback (buildFallbackCards + localizeCards), no D1

Includes fixes from 3 rounds of subagent code-review (tasks 16-30):
- CR1: autosave restructure, coerceOrientation, D1 comment cleanup
- CR2: fingerprint+serial+rollback+replay guard, idb post-open retry,
  enforceRetentionCap latent defense, sessionSlim absent invariant
- CR3: single-scene share guard (replaySourceRef), insert-beat fingerprint
  (beats.length), pass3 overflow double-count fix, detach gate unification

lib/engineClient.ts

@@ -10,6 +10,7 @@ import {
   resolveEngineConfig,
 } from "@/lib/clientModelConfig";
 import { loadClientTtsConfig } from "@/lib/clientTtsConfig";
+import { stripSessionVoices } from "@/lib/persistence/sessionSlim";
 import type {
   Character,
   FreeformClassifyRequest,
@@ -78,16 +79,11 @@ async function getJson<T>(path: string): Promise<T> {
 // data is bulky (~160KB/character via referenceAudioBase64) and the
 // scene-generation / vision / classify pipelines never need it — voices
 // are only consumed by /api/beat-audio, which receives them directly, not
-// via the session. So strip voices before transport.
+// via the session. So strip voices before transport. The stripping rule itself
+// lives in lib/persistence/sessionSlim.ts (shared with the local-store layer so
+// "what counts as voice" has one definition).
 function stripVoicesForTransport(session: Session): Session {
-  return {
-    ...session,
-    // Destructure voice out so the serialized payload drops the field
-    // entirely (voice is optional on Character), rather than serializing
-    // it as undefined/null. This is the ~160KB/character referenceAudioBase64
-    // we want off the wire on the server-fallback path.
-    characters: session.characters.map(({ voice: _voice, ...rest }) => rest),
-  };
+  return stripSessionVoices(session);
 }
 
 // The server strips voice from already-known characters before responding