feat(app-store): category filter bar and custom registry settings

- TemplateService: static LSIO_CATEGORY_MAP covers ~80 well-known apps
  across Automation, Downloaders, Media, Monitoring, Networking,
  Security, Development, Productivity, Utilities, and Other. Category
  lookup is O(1) and runs once at cache-fill time. Adds source field
  ('linuxserver' | 'custom') to Template for future per-source UX.
  Portainer v2 registries pass their native categories through unchanged.
  Adds clearCache() method wired to POST /api/templates/refresh-cache.

- AppStoreView: category pill bar (All + sorted dynamic categories)
  rendered between search and grid; active pill fills on click; clicking
  a category badge on a card also sets the active filter; app count
  updates reactively; filter composed with existing search query.

- SettingsModal: new App Store section (local-node only) exposes a
  custom Portainer v2 JSON URL input with Save & Refresh (saves setting
  then busts the 24h template cache) and Reset to Default.

backend/src/index.ts

@@ -1482,6 +1482,11 @@ app.get('/api/templates', async (req: Request, res: Response) => {
   }
 });
 
+app.post('/api/templates/refresh-cache', authMiddleware, (req: Request, res: Response) => {
+  templateService.clearCache();
+  res.json({ success: true });
+});
+
 app.post('/api/templates/deploy', async (req: Request, res: Response) => {
   try {
     const { stackName, template, envVars } = req.body;