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>

server/schemas.py

@@ -87,6 +87,15 @@ class FeatureCreate(FeatureBase):
     priority: int | None = None
 
 
+class FeatureUpdate(BaseModel):
+    """Request schema for updating a feature (partial updates allowed)."""
+    category: str | None = None
+    name: str | None = None
+    description: str | None = None
+    steps: list[str] | None = None
+    priority: int | None = None
+
+
 class FeatureResponse(FeatureBase):
     """Response schema for a feature."""
     id: int