fix(chat): ChatInterface's init keeps running after unmount, so a slow-init test crashes a later test's fallback #16274

ChatInterface.test.ts passes, 23/23. Each run also logs this error twice:

[ERROR] [ChatInterface] ❌ Chat initialization failed: TypeError: Cannot read properties of undefined (reading 'catch')
    at initializeChatInterface (ChatInterface.vue:1102)

It happens on base 65609f9, and also on a branch that carries #16265. The test stays green, so nothing surfaces it.

Cause (line numbers from base 65609f9)

1. components/__tests__/ChatInterface.test.ts:307, "shows loading state while fetching chat history", mocks initializeChatInterface with a 5 s delay (:310). It asserts synchronously and ends, and nothing awaits or unmounts that init.
2. components/chat/ChatInterface.vue:1054-1056 races the init against a 10 s setTimeout. onUnmounted (:1156) doesn't cancel it. The timer also isn't cleared when the init wins the race.
3. The timeout fires during a later test. The catch block then runs the fallback controller.loadChatSessions().catch(...) (:1102).
4. By that point mockReset: true (vitest.config.ts:75) has stripped :95's vi.fn().mockResolvedValue(undefined), so the call returns undefined and .catch throws. This is the class of bug the warning at vitest.config.ts:71 (Enhancement: document vitest mockReset interaction with vi.mock factories #3070) describes.

Production impact

This isn't only a test problem. An init that outlives its component still runs the fallback and writes to the chat store after unmount. On every successful mount, the 10 s timer is also left pending.

Acceptance criteria

• ChatInterface.vue clears the init timeout whether the init wins or loses the race.
• Once the component has unmounted, nothing after an await in the init runs. A test unmounts mid-init and asserts the fallback is never called.
• A run of ChatInterface.test.ts logs no Chat initialization failed.

Found while verifying #16253.