feat(pilot): ProposalBanner scaffold + Proposed state

New component that will replace the task-lane SuggestedFix card. Docks
above the chat composer with a 320ms slide-up animation. This commit
implements only the Proposed state (Tasks 8 & 9 fill Verifying, Partial,
AI-confirming, Nudge, Collapsed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/index.css

@@ -86,7 +86,12 @@
   --animate-slide-in-bottom: slide-in-from-bottom 200ms ease-out both;
   --animate-scale-in: scale-in 150ms ease-out both;
   --animate-fade: fadeIn 300ms ease both;
+  --animate-slide-up: slide-up 320ms cubic-bezier(.22,.9,.28,1) both;
 
+  @keyframes slide-up {
+    from { transform: translateY(14px); opacity: 0; }
+    to   { transform: translateY(0);    opacity: 1; }
+  }
   @keyframes fade-in {
     from { opacity: 0; } to { opacity: 1; }
   }