fix(i18n): replace hardcoded Chinese strings in Session Manager

- Add i18n keys for relative time (justNow, minutesAgo, hoursAgo, daysAgo)
- Add i18n keys for role labels (roleUser, roleSystem, roleTool)
- Add i18n keys for UI elements (tocTitle, searchSessions, clickToCopyPath)
- Update formatRelativeTime and getRoleLabel to accept t function
- Add useTranslation hook to SessionToc component

src/components/sessions/SessionMessageItem.tsx

@@ -59,7 +59,7 @@ export function SessionMessageItem({
       </Tooltip>
       <div className="flex items-center justify-between text-xs mb-1.5 pr-6">
         <span className={cn("font-semibold", getRoleTone(message.role))}>
-          {getRoleLabel(message.role)}
+          {getRoleLabel(message.role, t)}
         </span>
         {message.ts && (
           <span className="text-muted-foreground">