Replaces the dead ColorScheme.fromSeed(Colors.indigo) (declared but
never consumed by any screen/widget) with theme.dart: an explicit
dark ColorScheme plus a real AppStatusColors extension, since this
app genuinely uses success/warning status colors throughout (unlike
dufs-cloud's app). Routes all 10 screen/widget files' ~280 hardcoded
Colors.*/fontSize literals through it. Fixes the two flagged
DESIGN_AUDIT_TODO violations: home_screen's Resume/Start buttons now
use a real 2x horizontal:vertical padding ratio, and rep_circle's
BoxShadow (the one shadow in an otherwise flat dark UI) is removed.
