fix: diagnostic toggle hint, link_stats schema, pagination UX, rate-limit feedback

- inspector.html: collapsible section hint text now toggles between [expand]/[collapse] when clicked
- inspector.html: timeout and connection-loss during diagnostic poll now show a Retry button instead of a dead end
- inspector.html: 429 rate-limit response shows a clear human-readable message instead of generic error
- app.py: empty link_stats fallback now includes unifi_switches:{} for schema consistency with real data shape
- index.html: pagination overflow notice now says "export all as JSON" (opens in new tab) instead of misleadingly linking to raw API as navigation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -291,7 +291,7 @@ def api_links():
             return jsonify(json.loads(raw))
         except Exception as e:
             logger.error(f'Failed to parse link_stats JSON: {e}')
-    return jsonify({'hosts': {}, 'updated': None})
+    return jsonify({'hosts': {}, 'unifi_switches': {}, 'updated': None})
 
 
 @app.route('/api/events')