feat(ui): localize friendly agent errors and surface them as toasts

Thread a translate function through friendlyAgentErrorMessage/
friendlyProjectFailureMessage and move the hard-coded zh-CN error copy
into the i18n locale files (with a zh-CN fallback map). Generation
stream failures now report through a toast via showGenerationError in
addition to setError, so the removed inline .panel-error footer is
gone.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/styles.css

@@ -470,8 +470,7 @@ button:disabled {
   border-radius: 999px;
 }
 
-.inline-error,
-.panel-error {
+.inline-error {
   margin: 0;
   color: #b42318;
   font-size: 13px;
@@ -3743,10 +3742,6 @@ button:disabled {
   line-height: 1.62;
 }
 
-.panel-error {
-  padding: 0 14px 8px;
-}
-
 .canvas-model-popover {
   width: 348px;
 }