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_engine/src/lib.rs

@@ -7,6 +7,7 @@ pub mod card_animation;
 pub mod achievement_plugin;
 pub mod analytics_plugin;
 pub mod animation_plugin;
+pub mod avatar_plugin;
 pub mod auto_complete_plugin;
 pub mod audio_plugin;
 pub mod card_plugin;
@@ -123,6 +124,7 @@ pub use leaderboard_plugin::{LeaderboardPlugin, LeaderboardResource, Leaderboard
 pub use input_plugin::InputPlugin;
 pub use onboarding_plugin::{OnboardingPlugin, OnboardingScreen};
 pub use pause_plugin::{ForfeitConfirmScreen, PausePlugin, PauseScreen, PausedResource};
+pub use avatar_plugin::{AvatarFetchEvent, AvatarPlugin, AvatarResource};
 pub use profile_plugin::{ProfilePlugin, ProfileScreen};
 pub use radial_menu::{
     legal_destinations_for_card, radial_anchor_for_index, radial_hovered_index, RadialIcon,