feat(05-03): frontend fallback integration for research display

- ExerciseResearchPanel: add ProviderBadge component showing which AI
  tier (Ollama/Gemini/OpenRouter/OpenCode/Exa) served the response
- Add auto-retry on 429/503 with 2s delay and retry counter in button
- Normalize error messages for common failure modes (network, rate-limit)
- ResearchDisplay: pass onRetry prop to error state for inline retry button
- CSS: .research-panel-controls flex row, .provider-badge with per-provider
  colour coding, .rd-error-actions + .rd-retry button styles

frontend/src/components/ResearchDisplay.jsx

@@ -9,16 +9,23 @@ function ResearchLoadingSkeleton({ exerciseName }) {
   )
 }
 
-function ResearchError({ message, onDismiss }) {
+function ResearchError({ message, onDismiss, onRetry }) {
   return (
     <div className="rd-error" role="alert">
       <span className="rd-error-icon" aria-hidden="true">⚠</span>
       <span className="rd-error-message">{message}</span>
-      {onDismiss && (
-        <button className="rd-dismiss" onClick={onDismiss} aria-label="Dismiss error">
-          ×
-        </button>
-      )}
+      <div className="rd-error-actions">
+        {onRetry && (
+          <button className="rd-retry btn btn-sm btn-primary" onClick={onRetry}>
+            Retry
+          </button>
+        )}
+        {onDismiss && (
+          <button className="rd-dismiss" onClick={onDismiss} aria-label="Dismiss error">
+            ×
+          </button>
+        )}
+      </div>
     </div>
   )
 }
@@ -52,14 +59,15 @@ function ResearchSourceCard({ result, index }) {
  *   data      {object}   Research data: { summary, results }
  *   name      {string}   Exercise name (shown during loading)
  *   onDismiss {function} Clear error callback
+ *   onRetry   {function} Retry fetch callback
  */
-function ResearchDisplay({ loading, error, data, name, onDismiss }) {
+function ResearchDisplay({ loading, error, data, name, onDismiss, onRetry }) {
   if (loading) {
     return <ResearchLoadingSkeleton exerciseName={name} />
   }
 
   if (error) {
-    return <ResearchError message={error} onDismiss={onDismiss} />
+    return <ResearchError message={error} onDismiss={onDismiss} onRetry={onRetry} />
   }
 
   if (!data) return null