[feat] Add manager capability feature flags

Add support for manager v4 feature flag and client UI capability:
- MANAGER_SUPPORTS_V4: Server-side flag for v4 manager support
- supports_manager_v4_ui: Client-side flag for v4 UI support

These flags enable proper capability negotiation between frontend and
backend for manager UI selection (legacy vs v4).

Also fix TypeScript errors by adding @types/lodash.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/composables/useFeatureFlags.ts

@@ -7,7 +7,8 @@ import { api } from '@/scripts/api'
  */
 export enum ServerFeatureFlag {
   SUPPORTS_PREVIEW_METADATA = 'supports_preview_metadata',
-  MAX_UPLOAD_SIZE = 'max_upload_size'
+  MAX_UPLOAD_SIZE = 'max_upload_size',
+  MANAGER_SUPPORTS_V4 = 'extension.manager.supports_v4'
 }
 
 /**
@@ -21,6 +22,9 @@ export function useFeatureFlags() {
     },
     get maxUploadSize() {
       return api.getServerFeature(ServerFeatureFlag.MAX_UPLOAD_SIZE)
+    },
+    get supportsManagerV4() {
+      return api.getServerFeature(ServerFeatureFlag.MANAGER_SUPPORTS_V4)
     }
   })
 

src/config/clientFeatureFlags.json

@@ -1,3 +1,4 @@
 {
-  "supports_preview_metadata": true
+  "supports_preview_metadata": true,
+  "supports_manager_v4_ui": true
 }

tests-ui/tests/composables/useFeatureFlags.test.ts

@@ -59,6 +59,21 @@ describe('useFeatureFlags', () => {
       )
     })
 
+    it('should access supportsManagerV4', () => {
+      vi.mocked(api.getServerFeature).mockImplementation(
+        (path, defaultValue) => {
+          if (path === ServerFeatureFlag.MANAGER_SUPPORTS_V4) return true as any
+          return defaultValue
+        }
+      )
+
+      const { flags } = useFeatureFlags()
+      expect(flags.supportsManagerV4).toBe(true)
+      expect(api.getServerFeature).toHaveBeenCalledWith(
+        ServerFeatureFlag.MANAGER_SUPPORTS_V4
+      )
+    })
+
     it('should return undefined when features are not available and no default provided', () => {
       vi.mocked(api.getServerFeature).mockImplementation(
         (_path, defaultValue) => defaultValue as any
@@ -67,6 +82,7 @@ describe('useFeatureFlags', () => {
       const { flags } = useFeatureFlags()
       expect(flags.supportsPreviewMetadata).toBeUndefined()
       expect(flags.maxUploadSize).toBeUndefined()
+      expect(flags.supportsManagerV4).toBeUndefined()
     })
   })