fix(canvas): give the floating mode selector an opaque backdrop

Quill's outline button fill is `--muted` at 30%, so once the selector floated
over the feed the messages behind it showed through. Put an opaque `bg-card`
backdrop on the row at the button's own radius (`--radius-sm`), which leaves the
trigger's hover and expanded fills intact.

Generated-By: PostHog Code
Task-Id: e21b284f-a889-4f05-a683-8f2407961fa5

Author: adboio

packages/ui/src/features/canvas/components/ChannelHomeComposer.tsx

@@ -373,9 +373,11 @@ export const ChannelHomeComposer = forwardRef<
   return (
     <div className="relative flex w-full flex-col">
       {/* Canvas generation always runs in the cloud, so the local/cloud pick
-          doesn't apply while canvas mode is armed. */}
+          doesn't apply while canvas mode is armed. The row floats over the feed,
+          and the trigger's own fill is translucent, so it carries an opaque
+          backdrop at the button's radius to stop messages showing through. */}
       {!canvasArmed && (
-        <div className="absolute bottom-full left-0 mb-2 flex items-center gap-2">
+        <div className="absolute bottom-full left-0 mb-2 flex items-center gap-2 rounded-sm bg-card">
           <WorkspaceModeSelect
             value={workspaceMode}
             onChange={setWorkspaceMode}