feat(engine): wire avatar download and display into profile modal

- Add avatar_plugin: AvatarPlugin, AvatarResource, AvatarFetchEvent
  - After AvatarFetchEvent fires, spawns an async reqwest download task
  - On completion, decodes image bytes via image::load_from_memory →
    Image::from_dynamic and inserts into Assets<Image>
- Expand auth task to also call fetch_me_with_token immediately after
  login/register so avatar_url is available without a second round-trip
- poll_auth_task fires AvatarFetchEvent when avatar_url is Some, building
  the full URL from base_url + relative avatar path
- Profile modal shows 48px circular avatar ImageNode when AvatarResource
  is populated, or an initials disc (first letter of username) as fallback
- Add image = "0.25" and reqwest to solitaire_engine deps
- Add fetch_me_with_token helper to SolitaireServerClient for use when
  the access token hasn't been persisted to keychain yet

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_app/src/lib.rs

@@ -26,7 +26,7 @@ use bevy::winit::WinitWindows;
 use solitaire_data::{load_settings_from, provider_for_backend, settings_file_path, Settings};
 use solitaire_engine::{
     register_theme_asset_sources, AchievementPlugin, AnalyticsPlugin, AnimationPlugin, AssetSourcesPlugin,
-    AudioPlugin, AutoCompletePlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
+    AudioPlugin, AutoCompletePlugin, AvatarPlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
     CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, DifficultyPlugin, FeedbackAnimPlugin,
     FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
     OnboardingPlugin, PausePlugin, PlayBySeedPlugin, ProfilePlugin, ProgressPlugin,
@@ -187,6 +187,7 @@ pub fn run() {
         .add_plugins(HudPlugin)
         .add_plugins(HelpPlugin)
         .add_plugins(HomePlugin::default())
+        .add_plugins(AvatarPlugin)
         .add_plugins(ProfilePlugin)
         .add_plugins(PausePlugin)
         .add_plugins(SettingsPlugin::default())