Complete client UI i18n migration

Adds client-side i18n support, language settings, translated UI copy, and locale parity coverage.

packages/app/src/composer/submit.ts

@@ -1,3 +1,5 @@
+import { i18n } from "@/i18n/i18next";
+
 export type AgentInputSubmitResult = "noop" | "queued" | "submitted" | "failed";
 
 export interface AgentInputSubmitActionInput<TAttachment> {
@@ -17,6 +19,7 @@ export interface AgentInputSubmitActionInput<TAttachment> {
   setSendError: (message: string | null) => void;
   setIsProcessing: (isProcessing: boolean) => void;
   onSubmitError?: (error: unknown) => void;
+  failedToSendMessage?: string;
 }
 
 export async function submitAgentInput<TAttachment>(
@@ -67,7 +70,11 @@ export async function submitAgentInput<TAttachment>(
       input.setUserInput(trimmedMessage);
       input.setAttachments(attachments);
     }
-    input.setSendError(error instanceof Error ? error.message : "Failed to send message");
+    input.setSendError(
+      error instanceof Error
+        ? error.message
+        : (input.failedToSendMessage ?? i18n.t("composer.errors.failedToSend")),
+    );
     input.setIsProcessing(false);
     return "failed";
   }