feat: PATCH /api/mcp/servers/{name} — enable/disable toggle

Add `PATCH /api/mcp/servers/{name}` endpoint that accepts `{"enabled": bool}`,
updates `mcp_servers.<name>.enabled` in config.yaml, and calls `reload_config()`.
Mirrors the existing DELETE pattern.

Also wire the previously-defined-but-unrouted `_handle_mcp_server_delete` into
`handle_delete`, and `_handle_mcp_server_update` into a new `handle_put` +
`do_PUT` in server.py — fixing a pre-existing bug where those handlers existed
but were never reachable over HTTP.

UI: add a toggle button in each MCP server row in the system settings panel
(panels.js). Clicking it calls PATCH and reloads the list. Toggle button is
styled with `.mcp-toggle-enabled` / `.mcp-toggle-disabled` CSS classes. The
`toggle_supported` flag in the list response is now `True`.

i18n: add 5 new keys (`mcp_enable_server`, `mcp_disable_server`,
`mcp_enabled_toast`, `mcp_disabled_toast`, `mcp_toggle_failed`) to all 9
non-English locales (English values as placeholder translations).

Tests: add `TestMcpToggle` class with 7 tests covering disable, enable,
404-not-found, empty name, missing field, response payload, and URL-encoded name.
Update `test_empty_config` and visibility panel assertions to reflect
`toggle_supported: True` and the new toggle button in panels.js.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_issue696_mcp_visibility_panel.py

@@ -24,7 +24,8 @@ def test_mcp_panel_renders_status_badges_tool_counts_and_empty_error_states():
     assert "mcp-tool-count" in js
     assert "mcp-empty-state" in js
     assert "mcp-error-state" in js
-    assert "mcp_toggle_followup" in js
+    assert "toggleMcpServer" in js
+    assert "mcp-toggle-btn" in js
     assert "api('/api/mcp/servers')" in js
     assert "mcp-delete-btn" not in js
     assert "showMcpAddForm" not in js