feat: implemented plugin configuration UI and calendar event action slot

- Add configSchema support to plugin manifest and ServerPlugin registry
- Add schema-driven admin config page (string, secret, boolean, number, select fields)
- Add per-plugin config storage backend (JSON files + REST API)
- Add calendar-event-actions and admin-plugin-page slot names to plugin store
- Add registerCalendarEventAction and registerAdminPage to plugin API
- Add calendarFormHooks (onCalendarEventFormOpen/Save) to hook bus
- Add PluginSlot in calendar event modal for plugin action buttons
- Style calendar event action buttons to match Bulwark outline button design
- Add Configure link per plugin in admin plugins dashboard
- Add Jitsi Meet plugin with tests (repos/plugins/jitsi-meet)
- Exclude data/admin/plugins from ESLint (deployed plugin bundles)

stores/plugin-store.ts

@@ -21,6 +21,7 @@ import { usePolicyStore } from '@/stores/policy-store';
 const SLOT_NAMES: SlotName[] = [
   'toolbar-actions', 'email-banner', 'email-footer', 'composer-toolbar',
   'sidebar-widget', 'email-detail-sidebar', 'settings-section', 'context-menu-email', 'navigation-rail-bottom',
+  'calendar-event-actions', 'admin-plugin-page',
 ];
 
 function emptySlots(): Record<SlotName, SlotRegistration[]> {