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/error.rs

@@ -44,6 +44,8 @@ pub enum AppError {
     McpValidation(String),
     #[error("{0}")]
     Message(String),
+    #[error("HTTP {status}: {body}")]
+    HttpStatus { status: u16, body: String },
     #[error("{zh} ({en})")]
     Localized {
         key: &'static str,