feat: add feature editing capability for pending/in-progress features

Add the ability for users to edit features that are not yet completed,
allowing them to provide corrections or additional instructions when the
agent is stuck or implementing a feature incorrectly.

Backend changes:
- Add FeatureUpdate schema in server/schemas.py with optional fields
- Add PATCH /api/projects/{project_name}/features/{feature_id} endpoint
- Validate that completed features (passes=True) cannot be edited

Frontend changes:
- Add FeatureUpdate type in ui/src/lib/types.ts
- Add updateFeature() API function in ui/src/lib/api.ts
- Add useUpdateFeature() React Query mutation hook
- Create EditFeatureForm.tsx component with pre-filled form values
- Update FeatureModal.tsx with Edit button for non-completed features

The edit form allows modifying category, name, description, priority,
and test steps. Save button is disabled until changes are detected.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/hooks/useProjects.ts

@@ -4,7 +4,7 @@
 
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import * as api from '../lib/api'
-import type { FeatureCreate, ModelsResponse, Settings, SettingsUpdate } from '../lib/types'
+import type { FeatureCreate, FeatureUpdate, ModelsResponse, Settings, SettingsUpdate } from '../lib/types'
 
 // ============================================================================
 // Projects
@@ -94,6 +94,18 @@ export function useSkipFeature(projectName: string) {
   })
 }
 
+export function useUpdateFeature(projectName: string) {
+  const queryClient = useQueryClient()
+
+  return useMutation({
+    mutationFn: ({ featureId, update }: { featureId: number; update: FeatureUpdate }) =>
+      api.updateFeature(projectName, featureId, update),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['features', projectName] })
+    },
+  })
+}
+
 // ============================================================================
 // Agent
 // ============================================================================