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/test/domain/chat_timeline_test.dart

@@ -0,0 +1,120 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:intl/date_symbol_data_local.dart';
+import 'package:tane/domain/chat_timeline.dart';
+
+void main() {
+  // In the app, flutter_localizations loads intl's locale symbols; a pure test
+  // must initialize them itself before any DateFormat.
+  setUpAll(initializeDateFormatting);
+
+  PrivateMessage at(DateTime when) =>
+      PrivateMessage(fromPubkey: 'p', text: 't', at: when);
+
+  group('chatTimeline', () {
+    test('empty in, empty out', () {
+      expect(chatTimeline(const []), isEmpty);
+    });
+
+    test('a day separator precedes the first message of each day', () {
+      final items = chatTimeline([
+        at(DateTime(2026, 6, 14, 10)),
+        at(DateTime(2026, 6, 14, 11)),
+        at(DateTime(2026, 6, 15, 9)),
+      ]);
+      // sep, msg, msg, sep, msg
+      expect(items.map((i) => i is ChatDaySeparator), [
+        true,
+        false,
+        false,
+        true,
+        false,
+      ]);
+      expect((items[0] as ChatDaySeparator).day, DateTime(2026, 6, 14));
+      expect((items[3] as ChatDaySeparator).day, DateTime(2026, 6, 15));
+    });
+
+    test('messages stay oldest-first', () {
+      final items = chatTimeline([
+        at(DateTime(2026, 1, 1, 8)),
+        at(DateTime(2026, 1, 1, 9)),
+      ]);
+      expect(items.whereType<ChatMessageRow>(), hasLength(2));
+      expect(items.whereType<ChatDaySeparator>(), hasLength(1));
+    });
+  });
+
+  group('chatDayLabel', () {
+    final now = DateTime(2026, 6, 15, 12);
+
+    test('the two most recent days use the given today/yesterday labels', () {
+      expect(
+        chatDayLabel(
+          DateTime(2026, 6, 15),
+          now: now,
+          today: 'Today',
+          yesterday: 'Yesterday',
+          localeCode: 'en',
+        ),
+        'Today',
+      );
+      expect(
+        chatDayLabel(
+          DateTime(2026, 6, 14),
+          now: now,
+          today: 'Today',
+          yesterday: 'Yesterday',
+          localeCode: 'en',
+        ),
+        'Yesterday',
+      );
+    });
+
+    test('older days use a locale date, not today/yesterday', () {
+      final label = chatDayLabel(
+        DateTime(2026, 6, 10),
+        now: now,
+        today: 'Today',
+        yesterday: 'Yesterday',
+        localeCode: 'en',
+      );
+      expect(label, isNot(anyOf('Today', 'Yesterday')));
+      expect(label, contains('10')); // the day number
+    });
+
+    test('a previous year is shown', () {
+      final label = chatDayLabel(
+        DateTime(2024, 6, 10),
+        now: now,
+        today: 'Today',
+        yesterday: 'Yesterday',
+        localeCode: 'en',
+      );
+      expect(label, contains('2024'));
+    });
+
+    test('respects the locale (Spanish month name)', () {
+      final label = chatDayLabel(
+        DateTime(2026, 6, 10),
+        now: now,
+        today: 'Hoy',
+        yesterday: 'Ayer',
+        localeCode: 'es',
+      );
+      expect(label.toLowerCase(), contains('junio'));
+    });
+  });
+
+  group('chatBubbleTime', () {
+    test('English uses a 12-hour clock', () {
+      final s = chatBubbleTime(DateTime(2026, 1, 1, 14, 5), 'en');
+      expect(s.toUpperCase(), contains('PM'));
+    });
+
+    test('Spanish uses a 24-hour clock', () {
+      final s = chatBubbleTime(DateTime(2026, 1, 1, 14, 5), 'es');
+      expect(s, contains('14'));
+      expect(s.toUpperCase(), isNot(contains('PM')));
+    });
+  });
+}