fix: queue drain continues in background tabs and across chat switches

[samueljklee]

Problem

When the browser tab is not in focus, queued chat messages don't get sent. Two scenarios:

1. Tab not visible (switched to another app): The queue countdown uses recursive setTimeout(tick, 1000). Browsers aggressively throttle setTimeout in background tabs — after ~5 minutes, Chrome throttles to 1 per minute, and with Tab Freezing, JS can be completely paused. A 5-second countdown could take minutes or never fire.

2. Switched to another chat (same browser tab): When creating a new chat, the previous session's queue was actively cleared (clearSessionQueue(prevKey)), deleting all queued messages.

Changes

• Wall-clock countdown — startCountdown now uses Date.now() vs a fixed deadline instead of tick-decrement. Even if the browser throttles setTimeout from 1s to 60s, the next tick sees real wall time has elapsed and fires the callback immediately.

• Resume drain on tab focus — visibilitychange handler now calls tryDrainQueue() when the tab regains focus, catching any timers that were frozen while backgrounded.

• Preserve queue across sessions — Removed clearSessionQueue(prevKey) when creating a new chat. Queue items are preserved and drain when the user switches back.

• Clearer drain guards — Split the combined guard into two explicit checks: session-switched-away (preserves queue for later) vs execution-started (backs off).

Testing

1. Open a chat, send a message, while AI is executing queue another message
2. Switch to a different app/tab — queue should still drain after countdown
3. Switch to a different chat — queue should be preserved and drain when you switch back