v0.6.34

Show a signal in red when it is lost (stale / NaN)

Inputs now track the wall-clock time of their last VALID reading. When an input
that had data gets no valid reading within LOST_AFTER_SECONDS (5s) — the source
dropped out, or only "not available" frames arrive — the whole row (title, bar,
value, sparkline) renders in the theme's alarm red so a lost signal is obvious
at a glance. A NaN/inf reading is treated as invalid: it's ignored (the last
good value is kept) and the signal ages into the lost state. A signal that has
never reported stays in the dim "no data" look, not red.

The repaint tick flips inputs into/out of the lost look even when no frames
arrive, repainting only on a transition. _apply_update also skips None values
(dropped not-available fields) so the gauge keeps its last good reading.

401 tests green; ruff + mypy clean.