Create a new app silently fails with 404

**Summary:** Clicking "Create" in the "Create a new app" dialog POSTs to `/dev/apps/{name}/builder/save?tmp=true`, which returns `404 Not Found` when `adk web` falls back to `ApiServer`. The UI dismisses the dialog with no error feedback and the app is never created.

## Steps to Reproduce

1. Run `adk web` (without `python-multipart` installed)
2. Click the app selector dropdown
3. Click the `+` ("Create new agent") button
4. Enter a valid app name (e.g. `my_test_app`)
5. Click **Create**

## Expected Behavior

New app scaffold is created and the builder opens for `my_test_app`.

## Actual Behavior

Dialog dismisses silently. No app is created. Browser console shows:

Failed to load resource: the server responded with a status of 404 (Not Found)

Network trace:
POST /dev/apps/my_test_app/builder/save?tmp=true → 404 {"detail": "Not Found"}

Additionally, on every page load while an app is selected:
GET /dev/apps/{app}/build_graph → 404
GET /dev/apps/{app}/build_graph_image?dark_mode=false → 404
GET /dev/apps/{app}/build_graph_image?dark_mode=true → 404
GET /dev/apps/{app}/builder?ts=... → 404
GET /dev/apps/{app}/eval_sets → 404
GET /dev/apps/{app}/eval_results → 404

Server startup log emits:
WARNING - DevServer not found, falling back to ApiServer. Dev-only endpoints will not be available.
WARNING - python-multipart not installed. Builder UI endpoints will not be available.

## Screenshots

## Environment

| | |
|---|---|
| `google-adk` | 2.0.0 |
| Python | 3.12.12 |
| OS | macOS Darwin 24.2.0 |
| Command | `uv run adk web` |