feat(ui): add per-device appearance with a One Dark Pro dark theme (#68)

* feat(ui): add per-device appearance with a One Dark Pro dark theme

Language settings already own the other per-device UI choice; appearance
belongs next to it (system by default, light/dark to pin). Tokens remap
through the existing ink/paper/cloud variables so the rest of the app
does not grow dark: variants.

* fix(ui): remap leftover light chrome so dark mode stays One Dark Pro

Hardcoded whites, warm washes, and inset highlights were surviving
html.dark. They now go through tokens so light is unchanged and the
desktop chrome no longer flashes milky panels.

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Co-authored-by: Kelvin <kelvin@KelvindeMacBook-Pro.local>