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/db/chat_database.dart

@@ -0,0 +1,54 @@
+import 'package:drift/drift.dart';
+
+part 'chat_database.g.dart';
+
+/// A 1:1 chat message, held locally so a conversation survives leaving the
+/// screen and going offline.
+///
+/// This is deliberately NOT a [SyncColumns] inventory row: chat history is an
+/// ephemeral, per-device cache of what the relays delivered — not the user's
+/// CRDT-synced inventory. It never enters an inventory snapshot, backup, or the
+/// device-to-device sync, so it needs no HLC / tombstone / author metadata. It
+/// lives in its own encrypted database ([ChatDatabase]) to keep it fully
+/// isolated from the inventory schema, its migrations, and its sync stream.
+///
+/// [accountScope] namespaces rows per social identity (empty = the original
+/// identity), replacing the old per-key keystore prefix. The unique key makes
+/// de-duplication a DB invariant: a relay re-delivers the same gift wrap on
+/// every (re)subscribe, and an identical (scope, peer, sender, timestamp, text)
+/// row is simply not inserted twice.
+@TableIndex(
+  name: 'messages_by_conversation',
+  columns: {#accountScope, #peerPubkey, #sentAt},
+)
+class Messages extends Table {
+  IntColumn get id => integer().autoIncrement()();
+  TextColumn get accountScope => text()();
+  TextColumn get peerPubkey => text()();
+  TextColumn get fromPubkey => text()();
+  TextColumn get body => text()();
+
+  /// Milliseconds since epoch — the message's own timestamp (NIP-17), used for
+  /// ordering and for the read/unread mark.
+  IntColumn get sentAt => integer()();
+
+  @override
+  List<Set<Column>> get uniqueKeys => [
+    {accountScope, peerPubkey, fromPubkey, sentAt, body},
+  ];
+}
+
+/// The encrypted local chat cache (SQLCipher via an injected executor, exactly
+/// like [AppDatabase]). Separate database on purpose — see [Messages]. There is
+/// no historical schema to migrate: version 1, created fresh.
+@DriftDatabase(tables: [Messages])
+class ChatDatabase extends _$ChatDatabase {
+  ChatDatabase(super.e);
+
+  @override
+  int get schemaVersion => 1;
+
+  @override
+  MigrationStrategy get migration =>
+      MigrationStrategy(onCreate: (m) async => m.createAll());
+}