fix(chat): preserve session title against i18n MutationObserver overwrite

[tomekpanek]

Symptom

Click any historical session in the sidebar. The chat header briefly shows the saved title (e.g. "Polish Hello to Tomek"), then instantly flips back to "New Chat". Sending a message in that session works correctly (it routes to the right session), but the header stays stuck on "New Chat".

Affects every loaded session, every time. Also affects rename — typing a new title via the rename modal flashes the new value and reverts.

Root cause

<div class="chat-title" id="chat-title" data-i18n="auto.newChat2">New Chat</div> carries the data-i18n attribute. main.js installs a debounced MutationObserver that runs applyTranslations() on any DOM mutation — and applyTranslations reads data-i18n and overwrites textContent to the localized value of auto.newChat2.

Flow:

1. loadChatSession() sets titleEl.textContent = sessionTitle ✓
2. Mutation observer fires from the textContent change
3. applyTranslations() finds data-i18n="auto.newChat2" still on the element
4. It overwrites textContent back to "New Chat"

The session title is correct in memory (state._currentChatSession) and in the DB — only the header display is wrong.

Fix

Four sites need to manage data-i18n when changing the title:

Site	Action
loadChatSession — bind session title	removeAttribute('data-i18n') after setting textContent
newChatSession — reset to "New Chat"	setAttribute('data-i18n', 'auto.newChat2') so future locale switches still pick it up
Rename flow (renameChatSession)	removeAttribute('data-i18n') after setting new title
updateChatHeader	both branches — remove attr in the has-sid branch, restore in the no-sid branch

After the fix, the observer still runs but only finds data-i18n when the title is genuinely the i18n-managed "New Chat" placeholder. Real session titles are immune.

Test plan

• Click a historical session in the sidebar → its title shows and stays
• Click "+ New Chat" → header reverts to "New Chat"
• Switch UI language (Polish ↔ English) on a New Chat → label re-translates
• Switch UI language while a real session is open → its title does not get clobbered
• Rename a session via the modal → new title sticks, no flash-back

Verified

Tested on top of main (204e0e5, v3.6.0). Deterministic repro before, gone after.