Summary

Prevents EPUB reader reboot loops when css_rules.cache is missing, and stops Home from repeatedly retrying cover thumbnail generation for unsupported EPUB cover images.

Fixes #761.

Problem

Two issues that show up most often on ESP32-C3 with large EPUBs:

1. CSS cache miss can crash the reader

• If /.crosspoint/epub_<hash>/css_rules.cache is missing, we try to parse CSS on-demand.
• On large books / tight heap this can throw/abort and the device ends up reboot-looping.

2. Home periodically pauses with a "Loading..." popup

• Home attempts to generate thumb_<height>.bmp for recent book covers.
• Some EPUB covers are progressive JPEGs, which picojpeg doesn’t support (error 37 / PJPG_UNSUPPORTED_MODE).
• Since the thumb never appears, Home keeps retrying on subsequent renders → periodic pauses.

Solution

• CSS cache miss: fail open (continue without CSS) instead of crashing
  • Skip CSS parsing on cache-miss when the book is large or free heap is low
  • Guard best-effort CSS parsing so exceptions don’t crash the device
• Home thumbnails:
  • If JPEG decode fails, write a 0-byte thumb sentinel so Home stops retrying forever
  • Lyra theme: when cover is missing/invalid, render a simple placeholder with the book title in the cover box
• Metadata reads / recents migration: always skip CSS parsing (metadata only)

Notes / tradeoffs

• On books that hit the “skip CSS parse” path, rendering may be less styled (we intentionally skip building CSS rules to prioritize stability over perfect styling).
• For progressive JPEG covers, we intentionally stop retrying; users can convert the cover to baseline JPEG if they want the thumb.

Testing

On hardware (Xteink X4 / ESP32-C3):

• Delete /.crosspoint/epub_<hash>/css_rules.cache for a large EPUB, then open it → should not reboot; should log skipping CSS parse
• Return to Home repeatedly → should not keep retrying thumb generation for the same book/thumb size
• Verify Lyra Home shows title fallback when cover thumb is missing/invalid

Files changed

• lib/Epub/Epub.cpp
• src/RecentBooksStore.cpp
• src/components/themes/lyra/LyraTheme.cpp