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/pages/ExerciseEncyclopediaPage.css

@@ -563,3 +563,50 @@
     padding: var(--space-2) var(--space-3) var(--space-3);
   }
 }
+
+/* ============================================
+   PROVIDER BADGE — AI fallback indicator
+   ============================================ */
+
+.research-panel-controls {
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
+}
+
+.provider-badge {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  padding: 2px 8px;
+  border-radius: 9999px;
+  font-size: 11px;
+  font-weight: 600;
+  letter-spacing: 0.03em;
+  background: var(--bg-elevated);
+  border: 1px solid var(--border);
+  color: var(--text-muted);
+  white-space: nowrap;
+}
+
+.provider-local   { border-color: rgba(34, 197, 94, 0.4); color: #4ade80; }
+.provider-gemini  { border-color: rgba(99, 102, 241, 0.4); color: #818cf8; }
+.provider-openrouter { border-color: rgba(234, 179, 8, 0.4); color: #facc15; }
+.provider-opencode   { border-color: rgba(251, 146, 60, 0.4); color: #fb923c; }
+.provider-exa     { border-color: rgba(56, 189, 248, 0.4); color: #38bdf8; }
+.provider-unknown { border-color: var(--border); color: var(--text-muted); }
+
+/* Error actions row */
+.rd-error-actions {
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
+  flex-shrink: 0;
+}
+
+.rd-retry {
+  padding: 2px 10px;
+  font-size: var(--font-xs);
+  min-height: 28px;
+  border-radius: var(--radius-sm);
+}