refactor: extract queue state machine into testable JS module

[samueljklee]

Context

The message queue feature (merged in #22) has all its logic inlined in index.html (~6500 lines). The queue state machine — tryDrainQueue, startCountdown, cancelCountdown, pushToQueue, removeFromQueue, prependToQueue, drain state transitions, and the activeDelegatesRef counting — is tightly coupled to Preact hooks and refs, making it impossible to unit test the logic independently.

The current tests (94 of them) are static string-matching against the HTML file. They verify code patterns exist but can't test behavior — they can't confirm the countdown actually fires, that FIFO ordering is preserved, that the race condition retry works, or that session switching correctly preserves/restores queue state.

Proposal

Extract the queue state machine into a separate JS module (e.g. queue.js) that can be imported by index.html and independently unit tested.

What to extract

• Queue data operations: pushToQueue, removeFromQueue, prependToQueue, clearSessionQueue, getQueue — pure functions operating on a Map
• Drain state machine: tryDrainQueue, startCountdown, cancelCountdown, resumeQueue — the FSM transitions between idle → countdown → idle and paused → idle
• Drain guards: executingRef check, activeDelegatesRef check, activeKeyRef session guard, double-start prevention

What stays in index.html

• Preact component rendering (QueuePanel, InputArea modifications)
• SSE event handler wiring (calling the extracted functions)
• React state (queueVersion, countdownRemaining, executing)

Testing approach

Once extracted, use a lightweight test runner (vitest or plain Node.js assert) to test:

• FIFO ordering: push A, B, C → drain pops A first
• Drain guard: tryDrainQueue bails when executing=true
• Countdown lifecycle: start → tick → tick → callback fires
• Pause/resume: stop pauses, resume restarts
• Session isolation: queue for session A is independent of session B
• Re-queue on failure: prependToQueue puts item back at front
• Double-start prevention: tryDrainQueue with drainState=countdown is a no-op

Non-goals

• No build step / bundler — the module should work via <script type="module"> or a simple import pattern compatible with the current CDN-served architecture
• No framework change — Preact + htm stays as-is