Complete client UI i18n migration

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

packages/app/src/client-slash-commands/index.ts

@@ -8,6 +8,7 @@ export interface ClientSlashCommand {
   name: string;
   aliases: readonly string[];
   description: string;
+  descriptionKey: "composer.clientCommands.archiveAgent" | "composer.clientCommands.freshDraft";
   argumentHint: string;
   kind: ClientSlashCommandKind;
   execution: ClientSlashCommandExecution;
@@ -18,6 +19,7 @@ export const CLIENT_SLASH_COMMANDS: readonly ClientSlashCommand[] = [
     name: "exit",
     aliases: ["quit", "q"],
     description: "Archive the current agent",
+    descriptionKey: "composer.clientCommands.archiveAgent",
     argumentHint: "",
     kind: "archive-agent",
     execution: "immediate",
@@ -26,6 +28,7 @@ export const CLIENT_SLASH_COMMANDS: readonly ClientSlashCommand[] = [
     name: "clear",
     aliases: ["new"],
     description: "Archive this agent and start a fresh draft",
+    descriptionKey: "composer.clientCommands.freshDraft",
     argumentHint: "",
     kind: "replace-agent-with-draft",
     execution: "immediate",