feat: complete calendar integration + planner widget + 298 new KB intents

- Add calendar sync route to api.php (/api/calendar → calendar_sync.php)
- Add CALENDAR SYNC tab to admin with feed CRUD (add/edit/delete Google/ICS feeds)
- Add cal_sync_now action to admin for on-demand iCloud/Google sync
- Add cron: calendar_sync.php every 15 min (iCloud CalDAV + ICS feeds)
- Add PLANNER mini panel to index.html (left panel, shows today tasks + appointments)
- Update loadPlannerSummary() to render tasks/appts with priority dots and times
- Seed 298 new KB intents across 37 categories: science, history, tech, geography,
  math, health, food, space, philosophy, psychology, sports, music, film, travel,
  language, literature, finance, productivity, nature, facts, home automation,
  architecture, geopolitics, and more — 543 total intents

public_html/api.php

@@ -99,6 +99,9 @@ switch ($endpoint) {
     case "planner":
         require __DIR__ . '/../api/endpoints/planner.php';
         break;
+    case "calendar":
+        require __DIR__ . '/../api/endpoints/calendar_sync.php';
+        break;
     default:
         http_response_code(404);
         echo json_encode(['error' => 'Unknown endpoint: ' . $endpoint]);