Phase 006: migrate the chat page from naive-ui to Arco Design Vue

Swaps the rendering layer only. Chat store logic, keyboard handling, and the
/chat-run Socket.IO contract are untouched, so a mixed UI cannot hide a chat
regression behind a library swap.

Arco's dropdowns take their items from a #content slot of a-doption children
rather than a droplist prop, and Arco Triggers cannot be nested, so the two
toolbar menus lost their a-tooltip in favour of a native title. Both rules are
recorded in docs/arco-mapping.md, and tests/e2e/chat-input-dropdown.spec.ts plus
tests/e2e/chat-rendering-smoke.spec.ts keep the menus, the streaming markdown
rendering, and the no-Arco-inside-the-bubble red line from regressing silently.

docs/phases/phase_006.md

@@ -193,7 +193,7 @@ const message = useMessage()          // 我们的 composable，签名一致
 ```bash
 cd /Users/jack/source/MyProject/js/hermes-ui
 
-grep -c "from 'naive-ui'" packages/client/src/components/hermes/chat/*.vue \
+grep -cE "from ['\"]naive-ui['\"]" packages/client/src/components/hermes/chat/*.vue \
                          packages/client/src/views/hermes/ChatView.vue \
                          packages/client/src/stores/hermes/chat.ts
 # 期望：全部 0