fix(avatars): cache image handles so avatars don't flicker on redraw

iced's `image::Handle::from_bytes` assigns a fresh *unique* id on every call
(unlike `from_path`, which hashes). `avatar_view` built the handle inline in
`view()`, so every repaint produced a "new" image and iced re-uploaded the
texture each frame. Any redraw triggered it — notably the redraws fired on
mouse movement — so visible avatars flickered constantly while the mouse moved.

Fix: cache handles by a content hash of the PNG bytes (thread-local, UI thread)
and reuse the same `Handle` across redraws, giving a stable texture id. Covers
both presets and custom uploads.

Build + clippy clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -2915,6 +2915,33 @@ fn avatar_badge<'a>(name: &str, key: &str, size: f32) -> Element<'a, AppMessage>
         .into()
 }
 
+thread_local! {
+    /// Cache of avatar image handles, keyed by a hash of the PNG bytes, so the
+    /// SAME `image::Handle` (and thus the same GPU texture id) is reused across
+    /// redraws. `image::Handle::from_bytes` mints a fresh *unique* id on every
+    /// call, so building handles inline in `view()` made iced re-upload the
+    /// texture on every repaint — including the redraws fired on each mouse move —
+    /// which showed up as constant flicker. Lives on the (single) UI thread.
+    static AVATAR_HANDLE_CACHE: std::cell::RefCell<HashMap<u64, iced::widget::image::Handle>> =
+        std::cell::RefCell::new(HashMap::new());
+}
+
+/// A stable image handle for these exact PNG bytes (cached by content hash), so
+/// it keeps the same id across redraws — see [`AVATAR_HANDLE_CACHE`].
+fn cached_image_handle(bytes: bytes::Bytes) -> iced::widget::image::Handle {
+    use std::hash::{Hash, Hasher};
+    let mut hasher = std::collections::hash_map::DefaultHasher::new();
+    bytes.as_ref().hash(&mut hasher);
+    let key = hasher.finish();
+    AVATAR_HANDLE_CACHE.with(|cache| {
+        cache
+            .borrow_mut()
+            .entry(key)
+            .or_insert_with(|| iced::widget::image::Handle::from_bytes(bytes))
+            .clone()
+    })
+}
+
 /// Render a participant's avatar (W4): the chosen preset image if any, else the
 /// monogram fallback. `name`/`key` feed the monogram; `size` is the diameter.
 fn avatar_view<'a>(
@@ -2928,7 +2955,7 @@ fn avatar_view<'a>(
         .map(bytes::Bytes::from_static)
         .or_else(|| avatar.custom_png().map(bytes::Bytes::from));
     match png_bytes {
-        Some(b) => iced::widget::image(iced::widget::image::Handle::from_bytes(b))
+        Some(b) => iced::widget::image(cached_image_handle(b))
             .width(iced::Length::Fixed(size))
             .height(iced::Length::Fixed(size))
             .into(),