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/domain/chat_timeline.dart

@@ -0,0 +1,70 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:intl/intl.dart';
+
+/// One row of the rendered chat: either a day separator or a message. Pure data
+/// so the grouping can be unit-tested without widgets.
+sealed class ChatTimelineItem {
+  const ChatTimelineItem();
+}
+
+/// A "— Today —" style divider before the first message of a calendar day.
+class ChatDaySeparator extends ChatTimelineItem {
+  const ChatDaySeparator(this.day);
+
+  /// Local midnight of the day this divider introduces.
+  final DateTime day;
+}
+
+/// A chat bubble.
+class ChatMessageRow extends ChatTimelineItem {
+  const ChatMessageRow(this.message);
+
+  final PrivateMessage message;
+}
+
+/// Groups [messages] (oldest first) into a timeline, inserting a
+/// [ChatDaySeparator] before the first message of each local calendar day.
+/// Result stays oldest-first.
+List<ChatTimelineItem> chatTimeline(List<PrivateMessage> messages) {
+  final out = <ChatTimelineItem>[];
+  DateTime? lastDay;
+  for (final m in messages) {
+    final day = DateTime(m.at.year, m.at.month, m.at.day);
+    if (lastDay == null || day != lastDay) {
+      out.add(ChatDaySeparator(day));
+      lastDay = day;
+    }
+    out.add(ChatMessageRow(m));
+  }
+  return out;
+}
+
+/// The human label for a day separator: "Today"/"Yesterday" for the two most
+/// recent days (localized by the caller), otherwise a locale-formatted date
+/// (weekday + day + month, plus year when it differs from [now]).
+///
+/// [localeCode] must be an `intl`-known locale — pass the *Localizations* locale
+/// (which maps Asturian → Spanish), never the raw app locale, since `ast` has no
+/// `intl` date symbols and would throw. Mirrors backup_section.dart.
+String chatDayLabel(
+  DateTime day, {
+  required DateTime now,
+  required String today,
+  required String yesterday,
+  required String localeCode,
+}) {
+  final startOfToday = DateTime(now.year, now.month, now.day);
+  final startOfDay = DateTime(day.year, day.month, day.day);
+  final diffDays = startOfToday.difference(startOfDay).inDays;
+  if (diffDays == 0) return today;
+  if (diffDays == 1) return yesterday;
+  final format = day.year == now.year
+      ? DateFormat.MMMMEEEEd(localeCode)
+      : DateFormat.yMMMMEEEEd(localeCode);
+  return format.format(day);
+}
+
+/// The clock time shown on a bubble, in the locale's own 12/24-hour convention.
+/// See [chatDayLabel] for the [localeCode] caveat.
+String chatBubbleTime(DateTime at, String localeCode) =>
+    DateFormat.jm(localeCode).format(at);