feat: MSP-native language pass + conclude modal update (Phase 6)

- ConcludeSessionModal: "Conclude Session" → "Close Case"
- ChatSidebar: "New Chat" → "New Case", "chat history" → "case history"
- Language pass in AssistantChatPage already done in Phase 5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/assistant/ChatSidebar.tsx

@@ -68,7 +68,7 @@ export function ChatSidebar({
           className="flex-1 flex items-center justify-center gap-2 bg-primary text-white font-semibold text-sm rounded-lg px-4 py-2.5 hover:brightness-110 active:scale-[0.98] transition-all"
         >
           <Plus size={16} />
-          New Chat
+          New Case
         </button>
         {onToggleCollapse && (
           <button
@@ -154,7 +154,7 @@ export function ChatSidebarCollapsedBar({
       <button
         onClick={onExpand}
         className="flex items-center gap-1.5 rounded-md px-2.5 py-1.5 text-xs text-muted-foreground hover:text-heading hover:bg-elevated transition-colors"
-        title="Show chat history"
+        title="Show case history"
       >
         <History size={14} />
         <span>History</span>

frontend/src/components/assistant/ConcludeSessionModal.tsx

@@ -234,7 +234,7 @@ export function ConcludeSessionModal({
             </div>
             <div>
               <h2 className="text-base font-heading font-semibold text-foreground">
-                Conclude Session
+                Close Case
               </h2>
               <p className="text-xs text-muted-foreground truncate max-w-[300px]">
                 {chatTitle}