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>

linux/flutter/generated_plugin_registrant.cc

@@ -9,6 +9,7 @@
 #include <flutter_timezone/flutter_timezone_plugin.h>
 #include <open_file_linux/open_file_linux_plugin.h>
 #include <record_linux/record_linux_plugin.h>
+#include <sqlite3_flutter_libs/sqlite3_flutter_libs_plugin.h>
 #include <url_launcher_linux/url_launcher_plugin.h>
 
 void fl_register_plugins(FlPluginRegistry* registry) {
@@ -21,6 +22,9 @@ void fl_register_plugins(FlPluginRegistry* registry) {
   g_autoptr(FlPluginRegistrar) record_linux_registrar =
       fl_plugin_registry_get_registrar_for_plugin(registry, "RecordLinuxPlugin");
   record_linux_plugin_register_with_registrar(record_linux_registrar);
+  g_autoptr(FlPluginRegistrar) sqlite3_flutter_libs_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "Sqlite3FlutterLibsPlugin");
+  sqlite3_flutter_libs_plugin_register_with_registrar(sqlite3_flutter_libs_registrar);
   g_autoptr(FlPluginRegistrar) url_launcher_linux_registrar =
       fl_plugin_registry_get_registrar_for_plugin(registry, "UrlLauncherPlugin");
   url_launcher_plugin_register_with_registrar(url_launcher_linux_registrar);

linux/flutter/generated_plugins.cmake

@@ -6,6 +6,7 @@ list(APPEND FLUTTER_PLUGIN_LIST
   flutter_timezone
   open_file_linux
   record_linux
+  sqlite3_flutter_libs
   url_launcher_linux
 )