fix(ui): keep unread badges, mention toggles, and pending switches across a channel rename

[Lafnaps]

Rename a channel that has unread messages and its badge disappears. Reproducible on current main: let #foo accumulate unreads while you sit in #general, rename it to #bar from the tab — the badge is gone although the messages are still unread.

The channel_renamed handler in chat.js migrates DOM data-channel attributes, lastMessageDates, and activeChannel, but four pieces of state stay keyed under the old name: channelUnread (the count survives, invisible, under a key nothing reads anymore), _channelMentions (the per-channel mention toggle silently resets), channelList, and pendingChannelSwitch (a queued switch to the old name can never match again). The settings frame that accompanies a rename replaces channelList eventually, but it can't migrate unread/mention state it knows nothing about.

The handler now rekeys all four, retargets a pending switch to the new name so it can still resolve, and finishes with filterMessagesByChannel() + renderChannelTabs() so the tab re-renders with its migrated badge immediately. No protocol changes, no server changes; ?v= bumped for chat.js.

There's no JS harness in the repo, so this was verified against a live server with a scripted browser (fresh data dir, one registered agent posting into the renamed channel), running the same scenario on this branch and on main:

• three unreads in #foo (badge "3"), rename to #bar → badge "3" on the #bar tab; on main the badge disappears and the count stays stranded under foo.
• a mention toggle remembered for #foo is remembered for #bar after the rename; on main it resets.
• a pending switch aimed at #foo gets retargeted to #bar; on main it stays aimed at a name that no longer exists.
• no console errors on the patched page.

Python suite untouched by this change (static/ isn't imported): 81 passed before and after.