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)

lib/__tests__/plugin-store.test.ts

@@ -52,6 +52,8 @@ function resetStore() {
       'settings-section': [],
       'context-menu-email': [],
       'navigation-rail-bottom': [],
+      'calendar-event-actions': [],
+      'admin-plugin-page': [],
     },
     initialized: false,
   });