refactor: simplify thumbnail cache API and remove unused code

- ThumbnailDiskCache: combine get()+decodeThumbnail() into single load()
  method. Move bitmap resize logic into generateFromFile() so callers
  just pass a source file and the cache handles decode+resize+save.
- ProfilePictureFetcher: remove unused `options` field, remove @Stable
  annotation, delegate all resize logic to ThumbnailDiskCache.
- UserAvatar: fix stale doc comment.

https://claude.ai/code/session_01PBJS3HDrurLP3i5n4tMsCv

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/ui/components/UserAvatar.kt

@@ -61,7 +61,7 @@ data class ProfilePictureUrl(
  * @param contentDescription Accessibility description
  * @param loadProfilePicture Whether to load the profile picture (false = show robohash only)
  * @param loadRobohash Whether to generate robohash (false = show generic icon)
- * @param useThumbnailCache Whether to wrap the URL with profilepic:// scheme for thumbnail caching
+ * @param useThumbnailCache Whether to use the thumbnail disk cache for faster repeated loads
  */
 @Composable
 fun UserAvatar(