feat(chat): usable 1:1 chat — bottom-anchored, Drift-backed, dated

- Anchor the message list to the bottom (`reverse: true`) so new messages
  stay in view instead of landing below the fold.
- Move chat history from the OS keystore (O(n²) JSON blob, silent 200-msg
  cap) to a separate encrypted Drift/SQLCipher DB (`ChatDatabase`):
  indexed append, uncapped history, dedup as a unique-key invariant. It's
  an ephemeral per-device cache, isolated from the inventory schema, its
  migrations, and its sync. No data migration (pre-release).
- Add day separators (Today/Yesterday/locale date) and a per-bubble time,
  all via ICU (12/24h per locale; Localizations locale maps Asturian →
  Spanish for intl date symbols).
- Add peer avatars (deterministic colour from the pubkey + name initial),
  surface send failures that were previously silent, and make bubble text
  selectable (addresses, links).
- New i18n keys in en/es/pt/ast; tests for grouping, formatting, avatars,
  scroll anchoring, storage and send errors.

Docs: docs/design/chat-storage.md + open-decisions.md.

apps/app_seeds/lib/di/injector.dart

@@ -12,6 +12,7 @@ import 'package:path_provider/path_provider.dart';
 import '../data/species_catalog.dart';
 import '../data/species_repository.dart';
 import '../data/variety_repository.dart';
+import '../db/chat_database.dart';
 import '../db/database.dart';
 import '../db/encrypted_executor.dart';
 import '../i18n/strings.g.dart';
@@ -98,6 +99,12 @@ Future<void> configureDependencies() async {
     openEncryptedExecutor(await _databaseFile(), dbKeyHex),
   );
 
+  // Chat history lives in its own encrypted database (same SQLCipher key),
+  // isolated from the inventory schema/sync. See [Messages].
+  final chatDatabase = ChatDatabase(
+    openEncryptedExecutor(await _chatDatabaseFile(), dbKeyHex),
+  );
+
   // CRDT author id: a stable per-install slice of the root seed. Kept distinct
   // from the social-layer public key on purpose — unifying the two would rewrite
   // authorship of existing rows, a data-model decision for later.
@@ -116,7 +123,9 @@ Future<void> configureDependencies() async {
       deviceId: deviceId,
     );
   } catch (e, s) {
-    debugPrint('Social identity derivation failed; inventory-only mode: $e\n$s');
+    debugPrint(
+      'Social identity derivation failed; inventory-only mode: $e\n$s',
+    );
   }
 
   // Seed the bundled species catalog before the UI opens — but only once per
@@ -152,6 +161,7 @@ Future<void> configureDependencies() async {
   getIt
     ..registerSingleton<SecureKeyStore>(keyStore)
     ..registerSingleton<AppDatabase>(database)
+    ..registerSingleton<ChatDatabase>(chatDatabase)
     ..registerSingleton<SpeciesRepository>(speciesRepository)
     ..registerSingleton<VarietyRepository>(varietyRepository)
     ..registerSingleton<FileService>(fileService)
@@ -166,11 +176,14 @@ Future<void> configureDependencies() async {
     ..registerSingleton<OfferOutbox>(OfferOutbox(secretStore))
     // Per-identity stores are namespaced by the active account's scope.
     ..registerSingleton<MessageStore>(
-        MessageStore(secretStore, accountScope: scope))
+      MessageStore(chatDatabase, accountScope: scope),
+    )
     ..registerSingleton<ProfileStore>(
-        ProfileStore(secretStore, accountScope: scope))
+      ProfileStore(secretStore, accountScope: scope),
+    )
     ..registerSingleton<ProfileCache>(
-        ProfileCache(secretStore, accountScope: scope))
+      ProfileCache(secretStore, accountScope: scope),
+    )
     ..registerSingleton<ExportImportService>(
       ExportImportService(
         repository: varietyRepository,
@@ -292,21 +305,27 @@ Future<void> switchSocialAccount(int account) async {
   // Re-register the per-identity stores under the new scope, then the identity.
   getIt
     ..registerSingleton<MessageStore>(
-        MessageStore(secretStore, accountScope: scope))
+      MessageStore(getIt<ChatDatabase>(), accountScope: scope),
+    )
     ..registerSingleton<ProfileStore>(
-        ProfileStore(secretStore, accountScope: scope))
+      ProfileStore(secretStore, accountScope: scope),
+    )
     ..registerSingleton<ProfileCache>(
-        ProfileCache(secretStore, accountScope: scope))
+      ProfileCache(secretStore, accountScope: scope),
+    )
     ..registerSingleton<UnreadService>(
-        UnreadService(getIt<MessageStore>(), secretStore));
+      UnreadService(getIt<MessageStore>(), secretStore),
+    );
 
   final social = await SocialService.fromRootSeedHex(
     rootSeedHex,
     account: account,
     deviceId: deviceId,
   );
-  final connection =
-      SocialConnection(social: social, settings: getIt<SocialSettings>());
+  final connection = SocialConnection(
+    social: social,
+    settings: getIt<SocialSettings>(),
+  );
   final inbox = InboxService(
     connection: connection,
     selfPubkey: social.publicKeyHex,
@@ -342,6 +361,11 @@ Future<File> _databaseFile() async {
   return File(p.join(dir.path, 'tane_inventory.sqlite'));
 }
 
+Future<File> _chatDatabaseFile() async {
+  final dir = await getApplicationDocumentsDirectory();
+  return File(p.join(dir.path, 'tane_chat.sqlite'));
+}
+
 /// Picks the Tesseract language pack(s) for the current locale(s), limited to
 /// the bundled packs listed in `tessdata_config.json`. Prefers the app's chosen
 /// language, then the device's ordered locales.