ui: add AnimationMode to control GIF rendering behavior

Introduces an `AnimationMode` enum with `Reactive`, `Continuous`, and
`NoAnimation` variants to allow fine-grained control over GIF playback
across the UI. This supports performance optimizations and accessibility
features, such as disabling animations when requested.

- Plumbs AnimationMode through image rendering paths
- Replaces hardcoded gif frame logic with reusable `process_gif_frame`
- Supports customizable FPS in Continuous mode
- Enables global animation opt-out via `NoteOptions::NoAnimations`
- Applies mode-specific logic in profile pictures, posts, media carousels, and viewer

Animation behavior by context
-----------------------------

- Profile pictures: Reactive (render only on interaction/activity)
- PostView: NoAnimation if disabled in NoteOptions, else Continuous (uncapped)
- Media carousels: NoAnimation or Continuous (capped at 24fps)
- Viewer/gallery: Always Continuous (full animation)

In the future, we can customize these by power settings.

Signed-off-by: William Casarin <jb55@jb55.com>

crates/notedeck_ui/src/note/options.rs

@@ -35,6 +35,9 @@ bitflags! {
 
         /// Note has an unread reply indicator
         const UnreadIndicator = 1 << 16;
+
+        /// no animation override (accessibility)
+        const NoAnimations = 1 << 17;
     }
 }