fix(starlark): show installed apps with the other plugins, and let them toggle

An app installed from the store appeared nowhere and could not be enabled
or disabled. Same cause as the 404s: #253 surfaced installed apps in
/plugins/installed as `starlark:<app_id>` entries and routed `starlark:`
toggles to the Starlark manifest, and #330 removed both along with the
routes.

Without the first, the app store installs successfully into a list
nothing renders. Without the second, toggling one falls through to the
plugin_manager lookup and answers "Plugin not found" -- a Starlark app is
an entry in starlark-apps' own manifest, not a plugin in that sense.

Restored as two named helpers rather than the original inline blocks:

  _starlark_virtual_plugins()  reads the loaded plugin when there is one
                               and the on-disk manifest otherwise, so the
                               list is right before starlark-apps loads
  _toggle_starlark_app()       updates through the plugin's own
                               _update_manifest_safe when loaded, or the
                               manifest directly when not

Two changes on the original. The toggle now runs app_id through
_validate_and_sanitize_app_id first -- it reaches a filesystem manifest
and had no validation where the other Starlark routes all have it. And
_starlark_virtual_plugins swallows its own failures: the entries are
appended to the real plugin list, and a broken Starlark manifest should
cost the Starlark rows, not empty the plugins page.

6 new tests covering listing, the fields the UI keys on, toggle
persistence, the unknown-app 404, traversal rejection, and that a
Starlark failure leaves the rest of the list intact.

Full core suite: 3979 passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RRtqXDCnvnY6EQwhT5CV9

test/web_interface/test_starlark_pixlet_routes.py

@@ -199,3 +199,74 @@ class TestNoStarlarkRouteIsMissing:
             except NotFound:
                 missing.append(u)
         assert not missing, f"the frontend calls these and they are not registered: {missing}"
+
+
+class TestInstalledAppsAppearWithTheOtherPlugins:
+    """An installed .star app must be manageable like any other plugin.
+
+    #253 surfaced installed apps in /plugins/installed as `starlark:<app_id>`
+    entries, so they could be seen and enabled/disabled from the same list as
+    everything else, and routed `starlark:` toggles to the Starlark manifest.
+    #330 removed both. The result was an app that installs successfully, then
+    appears nowhere and cannot be turned on or off.
+    """
+
+    APPS = {'apps': {'quoteoftheday': {'name': 'A Quote A Day', 'enabled': True}}}
+
+    def test_an_installed_app_is_listed(self, client):
+        with patch('web_interface.blueprints.api_v3._get_starlark_plugin', return_value=None), \
+             patch('web_interface.blueprints.api_v3._read_starlark_manifest', return_value=self.APPS):
+            resp = client.get('/api/v3/plugins/installed')
+        ids = [p['id'] for p in resp.get_json()['data']['plugins']]
+        assert 'starlark:quoteoftheday' in ids, \
+            "an installed Starlark app does not appear among the plugins"
+
+    def test_the_entry_carries_what_the_ui_needs(self, client):
+        with patch('web_interface.blueprints.api_v3._get_starlark_plugin', return_value=None), \
+             patch('web_interface.blueprints.api_v3._read_starlark_manifest', return_value=self.APPS):
+            resp = client.get('/api/v3/plugins/installed')
+        entry = next(p for p in resp.get_json()['data']['plugins']
+                     if p['id'] == 'starlark:quoteoftheday')
+        assert entry['name'] == 'A Quote A Day'
+        assert entry['enabled'] is True
+        assert entry['is_starlark_app'] is True, "the UI keys its Starlark handling off this"
+        assert entry['category'] == 'Starlark App'
+
+    def test_a_starlark_failure_does_not_empty_the_plugin_list(self, client):
+        # The virtual entries are appended to the real ones; a broken manifest
+        # must cost the Starlark rows, not everybody else's.
+        with patch('web_interface.blueprints.api_v3._get_starlark_plugin',
+                   side_effect=RuntimeError('boom')):
+            resp = client.get('/api/v3/plugins/installed')
+        assert resp.status_code == 200
+        assert resp.get_json()['status'] == 'success'
+
+    def test_toggling_an_app_does_not_report_plugin_not_found(self, client):
+        written = {}
+        with patch('web_interface.blueprints.api_v3._get_starlark_plugin', return_value=None), \
+             patch('web_interface.blueprints.api_v3._read_starlark_manifest',
+                   return_value={'apps': {'quoteoftheday': {'enabled': True}}}), \
+             patch('web_interface.blueprints.api_v3._write_starlark_manifest',
+                   side_effect=lambda m: written.update(m) or True):
+            resp = client.post('/api/v3/plugins/toggle',
+                               json={'plugin_id': 'starlark:quoteoftheday', 'enabled': False})
+        body = resp.get_json()
+        assert body['status'] == 'success', body
+        assert body['enabled'] is False
+        assert written['apps']['quoteoftheday']['enabled'] is False, \
+            "the manifest was not actually updated"
+
+    def test_toggling_an_unknown_app_says_so(self, client):
+        with patch('web_interface.blueprints.api_v3._get_starlark_plugin', return_value=None), \
+             patch('web_interface.blueprints.api_v3._read_starlark_manifest',
+                   return_value={'apps': {}}):
+            resp = client.post('/api/v3/plugins/toggle',
+                               json={'plugin_id': 'starlark:nope', 'enabled': True})
+        assert resp.status_code == 404
+        assert 'nope' in resp.get_json()['message']
+
+    def test_a_traversal_app_id_is_rejected_before_touching_the_manifest(self, client):
+        resp = client.post('/api/v3/plugins/toggle',
+                           json={'plugin_id': 'starlark:../../etc/passwd', 'enabled': True})
+        assert resp.status_code == 400
+        assert 'invalid characters' in resp.get_json()['message']