Stop the export from hanging forever when the hidden film stalls #33

The render loop in exportPreview() (public/demo.mjs:318-343) can wait forever. If the queue is empty and ended is false, it calls film.play() and sleeps 250 ms (demo.mjs:321-322), with no upper limit. Two cases leave the button on "Rendering 42%" and disabled until the page is reloaded:

• film.onerror = reject (demo.mjs:294) only matters before loadeddata. A network or decode error later in the file is never seen, so the loop keeps polling.
• If film.play() keeps rejecting (the .catch(() => {}) at lines 321/333 hides this), no frame or ended event ever arrives.

Expected: a stalled export ends in "Export failed" and re-enables Download (the existing finally already cleans up).

Fix direction: set film.onerror again after loading, so it records the error in failure and calls wake(). Track the time of the last captured frame. If none has arrived for about 15 s while document.visibilityState === 'visible', set failure. Don't count time while the tab is hidden, since hidden tabs pause requestVideoFrameCallback.

Done when throttling the network to "Offline" mid-export in DevTools ends in "Export failed" within about 15 s, and normal exports are unchanged.