The browser build fetched its Japanese fallback font only for a Japanese UI,
so a Japanese design name, file name or typed text showed as tofu with the UI
in English. After drawing each frame the web shell now looks through the
painted text for kana, CJK ideographs and full-width forms while no CJK font
is loaded, and fetches the same font once on the first hit. A failed fetch
with an English UI leaves the language alone and is not retried in a loop.
Sessions that never show such text download nothing extra.
Fixes#720
Co-authored-by: Claude Opus 5.5 <[email protected]>