feat(avatars): 6 selectable preset avatars over presence — W4 Phase 2

Adds bundled preset avatars on top of the monogram foundation.

- New `Avatar` enum (Monogram | Preset(u8)) in the avatar module, serde-encoded;
  6 preset PNGs embedded via include_bytes! (placeholder art in assets/avatars/
  — swap for real designs later). +3 unit tests.
- `PeerState` gains `avatar` (rides the gossip presence plane like `sharing`);
  `AppConfig` gains `avatar` (persisted). Avatar choice flows app → core (Join +
  new SetAvatar command) → every self-state announce, so it reaches the room incl.
  late joiners, and changing it mid-call re-announces live.
- Settings "Avatar" section: monogram + 6 preset tiles, applied live + persisted.
- Rendering: `avatar_view` draws the chosen preset image (iced `image` feature,
  now enabled) else the monogram, in the self card, peer rows, and chat (chat
  looks up the sender's avatar from presence by id).

⚠️ BREAKING gossip wire change: presence Announce is signed (S2) and the
signature is recomputed by re-serializing the parsed struct, so a new PeerState
field means old and new builds can't verify each other's presence — ALL peers
must run a build >= this one (same as the S2 change). Redeploy dopedart before
2-machine testing.

Build + clippy clean, 222 lib + integration tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -176,6 +176,9 @@ pub struct AppConfig {
     /// Chosen UI colour theme.
     #[serde(default)]
     pub theme: AppTheme,
+    /// Our chosen avatar (W4): monogram fallback or a bundled preset.
+    #[serde(default)]
+    pub avatar: crate::avatar::Avatar,
     /// What a call recording captures (mixed / per-peer stems / both).
     #[serde(default)]
     pub recording_mode: RecordingMode,
@@ -233,6 +236,7 @@ impl Default for AppConfig {
             chat_drawer_width: default_chat_drawer_width(),
             room_layout: RoomLayout::default(),
             theme: AppTheme::default(),
+            avatar: crate::avatar::Avatar::default(),
             recording_mode: RecordingMode::default(),
             custom_sound_self_join: None,
             custom_sound_peer_join: None,