fix(ui): R10 S3c — Banner action buttons gain hover feedback

The accent + warning Banner action buttons had no hover state (flagged during S1 UAT).
Add the house hover:opacity-90 idiom. (The header-pill .glass-chrome consolidation is
deferred — 2 of the 3 pills already share the glass-menu/rounded-2xl look; unifying the
odd one, SyncStatus, needs a visual call on whether it becomes a full-height pill.)

frontend/src/components/Banner.tsx

@@ -8,12 +8,12 @@ const TONES: Record<BannerTone, { bar: string; icon: string; action: string }> =
   accent: {
     bar: "bg-accent/15 border-accent/30",
     icon: "text-accent",
-    action: "bg-accent text-accent-fg",
+    action: "bg-accent text-accent-fg hover:opacity-90",
   },
   warning: {
     bar: "bg-warn/15 border-warn/30",
     icon: "text-warn",
-    action: "bg-warn text-warn-fg",
+    action: "bg-warn text-warn-fg hover:opacity-90",
   },
 };