Land the UAT-approved backdrop direction as a real feature (replacing the dev
thumbnail prototype):
- 6 generated abstract SVG backdrops in public/backdrops/, one tuned to each
accent scheme (dark, mixed-colour, few light areas). Non-figurative + generated
= repo-safe, tiny, accent-matched.
- New data-backdrop="on" state (dark theme + the setting + not perf) paints the
per-scheme image on <body> (app root now transparent so it shows app-wide and
the glass refracts it) AND switches the glass to its translucent tier (the
user's tuned values: surface 50 / card 60 / scrim 30 / blur 10 / edge 25).
Off / light / perf -> flat colour + solid glass (the readable fallback).
- Settings > Appearance 'Background image' toggle (bgImage pref, dark-only),
trilingual. Flat is the fallback + opt-out.
- Remove the FeedBackdrop prototype + its tuner controls; tuner baseline resynced
to the glass-over-image tier, add a Backdrop-fade slider.
GOTCHA fixed: the dark-mode ambient 'background' shorthand reset background-size
to auto (image rendered un-covered); out-specified it with [data-theme="dark"].
