feat(offline): tier 2 phase 1 — Drift cache + read-through for notes

First slice of the tier 2 offline mode work tracked in Fable #147.
Notes are the only domain wired so far; this PR establishes the
shape and proves the round-trip end-to-end before extending to tasks
/ projects / events / etc. in phase 2.

Local store
- Adds drift ^2.20.0 + sqlite3_flutter_libs ^0.5.24 + path ^1.9.0
  runtime deps and drift_dev / build_runner dev deps.
- New lib/data/local/database.dart: FabledDatabase with CachedNotes
  (mirroring the Note model 1:1; tags stored as JSON-encoded text) and
  SyncMetadata (per-domain last_synced_at). Database file lives at
  $appDocs/fabled_cache.sqlite; opened lazily in a background isolate.
- fabledDatabaseProvider on the existing api_client_provider; closes
  the DB when the ProviderScope disposes.

Repository pattern
- NotesRepository now takes (NotesApi, FabledDatabase). Reads attempt
  the network first; on success the response is written to the cache.
  On NetworkException reads fall back to the cache (rethrowing if it
  is empty so fresh-install offline still surfaces the network error).
- Writes (create/update/delete) hit the server then sync the cache;
  offline write queueing is phase 3 and explicitly not wired here.
- AuthStatus.offline stays owned by AuthNotifier.verify()'s heartbeat;
  this repo deliberately doesn't poke that state.

OfflineBanner
- Reads notesRepository.lastSyncedAt(); when present, swaps
  "Offline — showing cached data." for "Offline — last sync X min
  ago." A 30s timer refreshes the relative-time string while the
  banner is mounted.
- Falls back to the generic message if no cache exists yet (fresh
  install offline).

Verification
- flutter analyze: No issues found

Out of scope (later phases per the task body)
- Tasks / projects / milestones / events / conversation list / journal
  day caching (phase 2 — same wrapper pattern, repeated)
- Generic write queue with conflict resolution on updated_at (phase 3)
- Read-only UI indicators on screens that depend on cached data
  (phase 4)
- Full offline chat (out of scope; needs a local model)
- RAG / search over cached content (out of scope)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

macos/Flutter/GeneratedPluginRegistrant.swift

@@ -13,6 +13,7 @@ import open_file_mac
 import package_info_plus
 import record_macos
 import shared_preferences_foundation
+import sqlite3_flutter_libs
 import url_launcher_macos
 
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
@@ -24,5 +25,6 @@ func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
   FPPPackageInfoPlusPlugin.register(with: registry.registrar(forPlugin: "FPPPackageInfoPlusPlugin"))
   RecordMacOsPlugin.register(with: registry.registrar(forPlugin: "RecordMacOsPlugin"))
   SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
+  Sqlite3FlutterLibsPlugin.register(with: registry.registrar(forPlugin: "Sqlite3FlutterLibsPlugin"))
   UrlLauncherPlugin.register(with: registry.registrar(forPlugin: "UrlLauncherPlugin"))
 }