feat(profile): profile photo or seed-illustration avatar

Profiles had only a coloured-initial disc. Now each person can set an
avatar — a real photo OR one of our own seed illustrations (so
pseudonymity stays the default; no photo required).

- ui/avatar.dart: the one-string value scheme carried in the kind:0
  'picture' — a 'data:' photo thumbnail, a 'tane:seed:<glyph>' token, or
  empty. The Nostr ProfileTransport already published 'picture'.
- Photos ride inline as a tiny thumbnail (reuses offerThumbnailDataUri,
  24 KB cap) — no media server, like offer photos.
- avatar_edit.dart: pick/take a photo, choose a seed illustration, or
  remove; ProfileStore stores it; ProfileScreen shows a big editable
  avatar and publishes it.
- PeerAvatar renders photo / illustration / initial fallback.
- ProfileCache gains picture/setPicture; the inbox caches peers' avatars
  alongside their names.
- i18n avatar block (en/es/pt/ast). Tests: value scheme + PeerAvatar
  render modes, store + cache round-trips.

Follow-up: render peers' avatars in chat/market/your-people (thread the
cached picture into PeerAvatar at each call site).

apps/app_seeds/lib/ui/avatar_edit.dart

@@ -0,0 +1,90 @@
+import 'package:flutter/material.dart';
+
+import '../i18n/strings.g.dart';
+import '../services/offer_thumbnail.dart' show offerThumbnailDataUri;
+import 'avatar.dart';
+import 'photo_pick.dart';
+import 'seed_glyph.dart';
+import 'theme.dart';
+
+/// Lets the user choose a profile avatar: take/pick a photo (shrunk to a tiny
+/// inline thumbnail), pick one of our seed illustrations, or remove it.
+///
+/// Returns the new avatar value (`data:` thumbnail or `tane:seed:<glyph>`),
+/// an empty string to clear it, or null when cancelled.
+Future<String?> showAvatarPicker(
+  BuildContext context, {
+  required String current,
+}) {
+  return showModalBottomSheet<String>(
+    context: context,
+    isScrollControlled: true,
+    builder: (sheetContext) {
+      final t = sheetContext.t;
+      return SafeArea(
+        child: Padding(
+          padding: const EdgeInsets.fromLTRB(16, 12, 16, 16),
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children: [
+              Text(t.avatar.title,
+                  style: Theme.of(sheetContext).textTheme.titleMedium),
+              const SizedBox(height: 8),
+              ListTile(
+                key: const Key('avatar.fromPhoto'),
+                contentPadding: EdgeInsets.zero,
+                leading: const Icon(Icons.add_a_photo_outlined,
+                    color: seedGreen),
+                title: Text(t.avatar.fromPhoto),
+                onTap: () async {
+                  final bytes = await pickPhoto(sheetContext);
+                  if (bytes == null) return;
+                  final uri = offerThumbnailDataUri(bytes, maxBytes: 24000);
+                  if (sheetContext.mounted) {
+                    Navigator.of(sheetContext).pop(uri ?? '');
+                  }
+                },
+              ),
+              const SizedBox(height: 6),
+              Text(t.avatar.illustration,
+                  style: const TextStyle(color: seedMuted, fontSize: 13)),
+              const SizedBox(height: 10),
+              Wrap(
+                spacing: 14,
+                runSpacing: 14,
+                children: [
+                  for (final name in avatarGlyphNames)
+                    InkWell(
+                      key: Key('avatar.glyph.$name'),
+                      borderRadius: BorderRadius.circular(40),
+                      onTap: () => Navigator.of(sheetContext)
+                          .pop(avatarGlyphToken(name)),
+                      child: CircleAvatar(
+                        radius: 24,
+                        backgroundColor: seedGreen,
+                        child: SeedGlyph(avatarGlyphChar(name)!,
+                            size: 26, color: Colors.white),
+                      ),
+                    ),
+                ],
+              ),
+              if (current.isNotEmpty) ...[
+                const SizedBox(height: 8),
+                Align(
+                  alignment: AlignmentDirectional.centerStart,
+                  child: TextButton.icon(
+                    key: const Key('avatar.remove'),
+                    onPressed: () => Navigator.of(sheetContext).pop(''),
+                    icon: const Icon(Icons.delete_outline, size: 18),
+                    label: Text(t.avatar.remove),
+                  ),
+                ),
+              ],
+            ],
+          ),
+        ),
+      );
+    },
+  );
+}