feat(settings): add enableLocalProxy toggle to gate main page proxy UI

New users often accidentally trigger ProxyToggle/FailoverToggle on the
main page. Add a settings toggle (default off) so the proxy controls
only appear when explicitly enabled. The proxy service start/stop in
settings remains independent of this visibility flag.

src/lib/schemas/settings.ts

@@ -14,6 +14,7 @@ export const settingsSchema = z.object({
   enableClaudePluginIntegration: z.boolean().optional(),
   skipClaudeOnboarding: z.boolean().optional(),
   launchOnStartup: z.boolean().optional(),
+  enableLocalProxy: z.boolean().optional(),
   language: z.enum(["en", "zh", "ja"]).optional(),
 
   // 设备级目录覆盖