fix: follow UDT's font size and live theme changes

UDT gives font sizes to keystone's fonts page, which writes
~/.config/keystone/fonts.json. deck-it hardcoded px sizes, so changing
the size on the fonts page did nothing on the tablet. theme.css now
carries --udt-font-size (override.deckit, else base, 8 to 40px, as
quickshell checks) on html, and deck.css sizes text in rem, scaled from
the old 14px design.

The page loaded theme.css once, so a scheme, wallpaper or size change
reached the tablet only after a reload. The shell now re-reads it every
5s and on visibility, swapping it into a <style> only when it changed,
and sets meta theme-color from --udt-base instead of Macchiato's
hardcoded base.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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