refactor(frontend): change API Settings from sidebar to horizontal tabs layout

Refactor ApiSettingsPage to use horizontal Tabs component like CodexLens page,
replacing the previous vertical sidebar navigation.

Changes:
- Replace Card-based sidebar with horizontal TabsList/TabsTrigger components
- Remove split-panel layout (sidebar + main panel)
- Use TabsContent to wrap each tab's content
- Import and use Tabs, TabsList, TabsTrigger, TabsContent from ui/Tabs
- Add api-settings.json import to en/zh locale index files for i18n support

Layout comparison:
- Before: Vertical sidebar (lg:w-64) with Card + nav buttons
- After: Horizontal tabs (TabsList) with tab triggers

ccw/frontend/src/locales/en/index.ts

@@ -24,6 +24,7 @@ import cliManager from './cli-manager.json';
 import cliMonitor from './cli-monitor.json';
 import mcpManager from './mcp-manager.json';
 import codexlens from './codexlens.json';
+import apiSettings from './api-settings.json';
 import theme from './theme.json';
 import executionMonitor from './execution-monitor.json';
 import cliHooks from './cli-hooks.json';
@@ -82,6 +83,7 @@ export default {
   ...flattenMessages(cliMonitor, 'cliMonitor'),
   ...flattenMessages(mcpManager, 'mcp'),
   ...flattenMessages(codexlens, 'codexlens'),
+  ...flattenMessages(apiSettings, 'apiSettings'),
   ...flattenMessages(theme, 'theme'),
   ...flattenMessages(cliHooks, 'cliHooks'),
   ...flattenMessages(executionMonitor, 'executionMonitor'),