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/ui/peer_avatar.dart

@@ -0,0 +1,59 @@
+import 'package:flutter/material.dart';
+
+/// A small round avatar for a chat peer. We have no profile photos yet, so it's
+/// a deterministic disc coloured from the peer's pubkey, showing the first
+/// letter of their [name] when known (else a person glyph). Same pubkey → same
+/// colour on every device, so the peer is visually recognizable.
+class PeerAvatar extends StatelessWidget {
+  const PeerAvatar({
+    required this.pubkey,
+    this.name,
+    this.radius = 14,
+    super.key,
+  });
+
+  final String pubkey;
+  final String? name;
+  final double radius;
+
+  @override
+  Widget build(BuildContext context) {
+    final letter = _initial(name);
+    return CircleAvatar(
+      radius: radius,
+      backgroundColor: peerAvatarColor(pubkey),
+      child: letter == null
+          ? Icon(Icons.person_outline, size: radius, color: Colors.white)
+          : Text(
+              letter,
+              style: TextStyle(
+                color: Colors.white,
+                fontSize: radius,
+                fontWeight: FontWeight.w600,
+              ),
+            ),
+    );
+  }
+
+  static String? _initial(String? name) {
+    if (name == null) return null;
+    final trimmed = name.trim();
+    if (trimmed.isEmpty) return null;
+    // characters.first handles emoji/combining marks safely.
+    return trimmed.characters.first.toUpperCase();
+  }
+}
+
+/// A deterministic, readable avatar colour derived from [pubkey]. Hashes the
+/// key to a hue, then fixes saturation/lightness so white text stays legible on
+/// top. Pure and stable — exposed for testing.
+Color peerAvatarColor(String pubkey) {
+  // FNV-1a over the code units — cheap, stable, well-spread across hues.
+  var hash = 0x811c9dc5;
+  for (final unit in pubkey.codeUnits) {
+    hash = (hash ^ unit) * 0x01000193;
+    hash &= 0xffffffff;
+  }
+  final hue = (hash % 360).toDouble();
+  return HSLColor.fromAHSL(1, hue, 0.45, 0.42).toColor();
+}