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.dart

@@ -0,0 +1,40 @@
+import 'seed_glyph.dart';
+
+/// A profile avatar is carried in one string (stored locally and published as
+/// the NIP-01 kind:0 `picture`), in one of three shapes:
+///  - `data:image/jpeg;base64,…` — a tiny photo thumbnail (rides inline, no
+///    media server, like offer photos);
+///  - `tane:seed:<name>` — one of our own seed illustrations (for people who'd
+///    rather not put a real photo — pseudonymity stays the default);
+///  - empty — the deterministic coloured disc with the name's initial.
+const avatarGlyphPrefix = 'tane:seed:';
+
+/// The seed illustrations offered as ready-made avatars. Names are the stable
+/// storage key (appended to [avatarGlyphPrefix]); the glyphs already ship in
+/// [SeedGlyphs]. Order is the picker order.
+const avatarGlyphNames = <String>[
+  'jars',
+  'sack',
+  'jar',
+  'scattered',
+  'pouring',
+  'mug',
+  'bigSpoon',
+  'smallSpoon',
+];
+
+/// The glyph char for an avatar illustration [name], or null if unknown.
+String? avatarGlyphChar(String name) => switch (name) {
+      'jars' => SeedGlyphs.jars,
+      'sack' => SeedGlyphs.sack,
+      'jar' => SeedGlyphs.jar,
+      'scattered' => SeedGlyphs.scattered,
+      'pouring' => SeedGlyphs.pouring,
+      'mug' => SeedGlyphs.mug,
+      'bigSpoon' => SeedGlyphs.bigSpoon,
+      'smallSpoon' => SeedGlyphs.smallSpoon,
+      _ => null,
+    };
+
+/// The token that selects illustration [name] (e.g. `tane:seed:jars`).
+String avatarGlyphToken(String name) => '$avatarGlyphPrefix$name';