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-tauri/src/commands/proxy.rs

@@ -253,6 +253,19 @@ pub async fn switch_proxy_provider(
     app_type: String,
     provider_id: String,
 ) -> Result<(), String> {
+    // Block official providers during proxy takeover
+    let provider = state
+        .db
+        .get_provider_by_id(&provider_id, &app_type)
+        .map_err(|e| format!("读取供应商失败: {e}"))?
+        .ok_or_else(|| format!("供应商不存在: {provider_id}"))?;
+    if provider.category.as_deref() == Some("official") {
+        return Err(
+            "代理接管模式下不能切换到官方供应商 (Cannot switch to official provider during proxy takeover)"
+                .to_string(),
+        );
+    }
+
     state
         .proxy_service
         .switch_proxy_target(&app_type, &provider_id)