Start button stays active during running state

[ayyazzafar]

Reproduction:

1. Open index.html
2. Click Start — timer begins counting down
3. Click Start again

Expected: the second click is a no-op (or the button is visually disabled).
Actual: start() early-returns because intervalId is already set, which is correct logic-wise, but the button remains clickable and gives no feedback. Users have reported clicking Start repeatedly thinking it isn't responding.

Fix proposal: toggle a disabled attribute on #startBtn once a session is running, and re-enable it on stop() or reset().