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/Cargo.toml

@@ -6,6 +6,8 @@ edition.workspace = true
 
 [dependencies]
 bevy           = { workspace = true }
+image          = { workspace = true }
+reqwest        = { workspace = true }
 kira           = { workspace = true }
 solitaire_core = { workspace = true }
 solitaire_data = { workspace = true }