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/i18n/strings_pt.g.dart

@@ -769,6 +769,9 @@ class _Translations$chat$pt extends Translations$chat$en {
 	@override String get offline => 'Configura a partilha para enviar mensagens';
 	@override String get payG1 => 'Pagar em Ğ1';
 	@override String get g1Copied => 'Endereço Ğ1 copiado — cola-o na tua carteira';
+	@override String get today => 'Hoje';
+	@override String get yesterday => 'Ontem';
+	@override String get sendError => 'Não foi possível enviar — verifica a tua ligação';
 }
 
 // Path: trust
@@ -1618,6 +1621,9 @@ extension on TranslationsPt {
 			'chat.offline' => 'Configura a partilha para enviar mensagens',
 			'chat.payG1' => 'Pagar em Ğ1',
 			'chat.g1Copied' => 'Endereço Ğ1 copiado — cola-o na tua carteira',
+			'chat.today' => 'Hoje',
+			'chat.yesterday' => 'Ontem',
+			'chat.sendError' => 'Não foi possível enviar — verifica a tua ligação',
 			'trust.none' => 'Ainda ninguém os avaliza',
 			'trust.count' => ({required Object n}) => 'Avalizada por ${n}',
 			'trust.vouch' => 'Conheço esta pessoa',