feat: classify stream check errors with color-coded toasts

Distinguish between "provider rejects probe" (yellow warning) and
"genuinely broken" (red error) in health check results.

Backend: add AppError::HttpStatus variant to carry structured HTTP
status codes, populate http_status on error results, classify codes
into short labels (e.g. "Auth rejected (401)"), and truncate overly
long response bodies.

Frontend: route 401/403/400/429/5xx to toast.warning with localized
hints explaining the error may not indicate actual unusability; route
404/402/connection errors to toast.error. Add i18n keys for all three
locales (zh/en/ja).

Also deduplicate check_once by reusing build_stream_check_result.

src-tauri/src/commands/stream_check.rs

@@ -116,15 +116,24 @@ pub async fn stream_check_all_providers(
             claude_api_format_override,
         )
         .await
-        .unwrap_or_else(|e| StreamCheckResult {
-            status: HealthStatus::Failed,
-            success: false,
-            message: e.to_string(),
-            response_time_ms: None,
-            http_status: None,
-            model_used: String::new(),
-            tested_at: chrono::Utc::now().timestamp(),
-            retry_count: 0,
+        .unwrap_or_else(|e| {
+            let (http_status, message) = match &e {
+                crate::error::AppError::HttpStatus { status, .. } => (
+                    Some(*status),
+                    StreamCheckService::classify_http_status(*status).to_string(),
+                ),
+                _ => (None, e.to_string()),
+            };
+            StreamCheckResult {
+                status: HealthStatus::Failed,
+                success: false,
+                message,
+                response_time_ms: None,
+                http_status,
+                model_used: String::new(),
+                tested_at: chrono::Utc::now().timestamp(),
+                retry_count: 0,
+            }
         });
 
         let _ = state