Add /-/actions endpoint to list registered actions

This adds a new endpoint at /-/actions that lists all registered actions
in the permission system. The endpoint supports both JSON and HTML output.

Changes:
- Added _actions() method to Datasette class to return action list
- Added route for /-/actions with JsonDataView
- Created actions.html template for nice HTML display
- Added template parameter to JsonDataView for custom templates
- Moved respond_json_or_html from BaseView to JsonDataView
- Added test for the new endpoint

The endpoint requires view-instance permission and provides details about
each action including name, abbreviation, description, resource class,
and parent/child requirements.

Closes #2547

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

tests/test_html.py

@@ -1176,3 +1176,13 @@ async def test_custom_csrf_error(ds_client):
     assert response.status_code == 403
     assert response.headers["content-type"] == "text/html; charset=utf-8"
     assert "Error code is FORM_URLENCODED_MISMATCH." in response.text
+
+
+@pytest.mark.asyncio
+async def test_actions_page(ds_client):
+    response = await ds_client.get("/-/actions")
+    assert response.status_code == 200
+    assert "Registered Actions" in response.text
+    assert "<th>Name</th>" in response.text
+    assert "view-instance" in response.text
+    assert "view-database" in response.text