Summary

Two related UX/stability issues on ESP32-C3 when working with EPUBs:

• Reader can crash-loop when css_rules.cache is missing and we attempt on-demand CSS parsing.
• Home can periodically pause because it keeps retrying cover thumbnail generation when the EPUB cover JPEG is unsupported.

Tracking fix in #762.

Details

1) Reader crash loop on CSS cache miss

• Missing: /.crosspoint/epub_<hash>/css_rules.cache
• We attempt to parse CSS on-demand.
• On large books / tight heap this can throw/abort → reboot loop.

2) Home "Loading..." pauses from repeated thumbnail retries

• Home tries to generate thumb_<height>.bmp for recent books.
• Progressive JPEG covers fail in picojpeg with error 37 (PJPG_UNSUPPORTED_MODE).
• Without a thumb, Home keeps retrying on later renders, causing visible pauses.

Repro (high level)

• Delete/miss css_rules.cache for a large EPUB, open it in Reader → crash loop.
• Use an EPUB with a progressive JPEG cover, go to Home repeatedly → repeated thumb attempts.

Expected

• Reader should fail open (continue without CSS) instead of abort/reboot.
• Home should not repeatedly retry thumbnail generation for unsupported covers.