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>

Cargo.lock

@@ -4034,9 +4034,14 @@ checksum = "85ab80394333c02fe689eaf900ab500fbd0c2213da414687ebf995a65d5a6104"
 dependencies = [
  "bytemuck",
  "byteorder-lite",
+ "color_quant",
+ "gif",
+ "image-webp",
  "moxcms",
  "num-traits",
  "png 0.18.1",
+ "zune-core",
+ "zune-jpeg",
 ]
 
 [[package]]
@@ -7013,8 +7018,10 @@ dependencies = [
  "bevy",
  "chrono",
  "dirs",
+ "image",
  "jni 0.21.1",
  "kira",
+ "reqwest",
  "resvg",
  "ron",
  "serde",