Add global polyfills for AbortSignal.timeout() (Safari <17, Firefox <122)
and crypto.randomUUID() (Safari <15.4, Firefox <95) in shell HTML templates.
Add -webkit-backdrop-filter prefix across 13 files for older Safari support.
Add Firefox scrollbar (scrollbar-width/scrollbar-color), range input
(::-moz-range-thumb/track), and color-mix() rgba fallbacks. Create shared
compat.ts utility module. Lowers browser floor from Safari 17/Firefox 122
to Safari 15.4/Firefox 95.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
