Ellipsize the shimmer overlay and center the stop icon

The shimmer ::before repeats the full data-shimmer-text, so on
truncating labels it painted the clipped characters over the base
text's ellipsis. The overlay now truncates identically (same font and
box, so the ellipsis lands in the same spot).

The stop square sized its svg directly, which competed with the
button's [&_svg]:size-4 rule and sat the glyph off-center — the size
override now lives on the button so class merge resolves it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

desktop/src/features/messages/ui/MessageComposerToolbar.tsx

@@ -322,13 +322,16 @@ export const MessageComposerToolbar = React.memo(
           {onStopAgent && !isSending ? (
             <Button
               aria-label="Stop agent"
-              className="rounded-full"
+              // Icon size set here so it wins the base [&_svg]:size-4 via
+              // class merge — sizing the svg directly leaves both rules
+              // competing and the glyph off-center.
+              className="rounded-full [&_svg]:size-3.5"
               data-testid="stop-agent"
               onClick={onStopAgent}
               size="icon"
               type="button"
             >
-              <Square aria-hidden className="h-3.5 w-3.5 fill-current" />
+              <Square aria-hidden className="fill-current" />
             </Button>
           ) : (
             <Button

desktop/src/shared/styles/globals/animations.css

@@ -165,6 +165,13 @@
 .buzz-shimmer::before,
 .shimmer::before {
   animation: buzz-shimmer var(--buzz-shimmer-duration) linear infinite;
+  /* Truncate exactly like the base text: the overlay repeats the FULL
+     data-shimmer-text, and without its own ellipsis it paints the clipped
+     characters right over the base ellipsis on truncating labels (sidebar
+     titles). Same font + box → the ellipsis lands in the same spot. */
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
   background-clip: text;
   background-image: linear-gradient(
     90deg,