fix(proxy): resolve 404 error and auto-setup proxy targets

Issues fixed:
1. Route prefix mismatch - Live config was set to use /claude, /codex,
   /gemini prefixes but server only had routes without prefixes
2. Proxy targets not auto-configured - start_with_takeover only modified
   Live config but didn't set is_proxy_target=true for current providers

Changes:
- Add prefixed routes (/claude/v1/messages, /codex/v1/*, /gemini/v1beta/*)
  to server.rs for backward compatibility
- Remove URL prefixes from takeover_live_configs() - server can route
  by API endpoint path alone (/v1/messages=Claude, /v1/chat/completions=Codex)
- Add setup_proxy_targets() method that automatically sets current providers
  as proxy targets when starting proxy with takeover
- Unify proxy toggle in SettingsPage to use takeover mode (same as main UI)
- Fix error message extraction in useProxyStatus hooks
- Fix provider switch logic to check both takeover flag AND proxy running state

src/components/settings/SettingsPage.tsx

@@ -174,8 +174,8 @@ export function SettingsPage({
 
   const {
     isRunning,
-    start: startProxy,
-    stop: stopProxy,
+    startWithTakeover: startProxy,
+    stopWithRestore: stopProxy,
     isPending: isProxyPending,
   } = useProxyStatus();
   const [failoverEnabled, setFailoverEnabled] = useState(true);