feat: block official provider switching during proxy takeover

Prevent users from switching to official providers (Anthropic/OpenAI/Google)
when proxy takeover is active, as using a proxy with official APIs may cause
account bans.

Defense-in-depth across 4 layers:
- Backend: ProviderService::switch(), hot_switch_provider(), switch_proxy_provider command
- Frontend: useProviderActions soft guard with error toast
- UI: ProviderActions button disabled with ShieldAlert icon
- Tray menu: official provider items disabled with ⛔ indicator

Also warns when enabling proxy takeover while current provider is official.

src/components/providers/ProviderCard.tsx

@@ -175,6 +175,8 @@ export function ProviderCard({
 
   const usageEnabled = provider.meta?.usage_script?.enabled ?? false;
   const isOfficial = isOfficialProvider(provider, appId);
+  const isOfficialBlockedByProxy =
+    isProxyTakeover && (provider.category === "official" || isOfficial);
   const isCopilot =
     provider.meta?.providerType === PROVIDER_TYPES.GITHUB_COPILOT ||
     provider.meta?.usage_script?.templateType === "github_copilot";
@@ -424,6 +426,7 @@ export function ProviderCard({
               isInConfig={isInConfig}
               isTesting={isTesting}
               isProxyTakeover={isProxyTakeover}
+              isOfficialBlockedByProxy={isOfficialBlockedByProxy}
               isOmo={isAnyOmo}
               onSwitch={() => onSwitch(provider)}
               onEdit={() => onEdit(provider)}