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/ui/peer_avatar_test.dart

@@ -0,0 +1,38 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/ui/peer_avatar.dart';
+
+void main() {
+  group('peerAvatarColor', () {
+    test('is deterministic for the same pubkey', () {
+      expect(peerAvatarColor('ab' * 32), peerAvatarColor('ab' * 32));
+    });
+
+    test('differs for different pubkeys', () {
+      expect(peerAvatarColor('ab' * 32), isNot(peerAvatarColor('cd' * 32)));
+    });
+
+    test('is fully opaque (legible disc)', () {
+      expect(peerAvatarColor('feed').a, 1.0);
+    });
+  });
+
+  group('PeerAvatar', () {
+    Widget host(Widget child) => MaterialApp(
+      home: Scaffold(body: Center(child: child)),
+    );
+
+    testWidgets('shows the name initial when known', (tester) async {
+      await tester.pumpWidget(host(PeerAvatar(pubkey: 'aa', name: 'rosa')));
+      expect(find.text('R'), findsOneWidget); // uppercased first letter
+    });
+
+    testWidgets('falls back to a glyph when the name is unknown', (
+      tester,
+    ) async {
+      await tester.pumpWidget(host(const PeerAvatar(pubkey: 'aa')));
+      expect(find.byIcon(Icons.person_outline), findsOneWidget);
+      expect(find.byType(Text), findsNothing);
+    });
+  });
+}