feat(web): group plugin pages by plugin in sidebar with collapsible sections

- Group pages by plugin when a plugin has multiple pages, collapse under
  the plugin label; single-page plugins render directly without nesting
- Rename "Extension Pages" to "Plugin Pages" with tooltip explaining
  these are visual pages provided by installed plugins
- Add pluginLabel to PluginPageItem for display

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/src/i18n/locales/en-US.ts

@@ -5,7 +5,8 @@ const enUS = {
     installedPlugins: 'Installed Plugins',
     pluginMarket: 'Marketplace',
     mcpServers: 'MCP Servers',
-    pluginPages: 'Extension Pages',
+    pluginPages: 'Plugin Pages',
+    pluginPagesTooltip: 'Visual pages provided by installed plugins',
     quickStart: 'Quick Start',
   },
   common: {
@@ -1294,8 +1295,8 @@ const enUS = {
     },
   },
   pluginPages: {
-    selectFromSidebar: 'Select an extension page from the sidebar',
-    invalidPage: 'Invalid extension page',
+    selectFromSidebar: 'Select a plugin page from the sidebar',
+    invalidPage: 'Invalid plugin page',
   },
 };