Auto dark/bright takes a light sensor where the device allows one

lib/theme/light.dart: dark at 15 lux or less, bright from 60, held 5 s; Settings shows the
reading. The Redmi Pad SE refuses apps its only light sensor (Xiaomi's, from the front
camera), so there Auto keeps following Android's dark theme. DASH-NEXT.md records the
other lux sources.

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

lib/README.md

@@ -41,7 +41,8 @@ pirate-station-aide's (main views, then the rarely used ones grouped as Tools).
   and the state gradient `good` → `caution` → `alarm`; plus the derived `surface` (raised bars) and
   `onFill(colour)` (text on a fill). No widget names a colour. Themes are JSON in `assets/themes/`, `rei` by
   default; smart-serow's four-key files still load (their `highlight` is `alarm`).
-- **Dark or bright.** `Themes.mode` is `auto` (Android's dark theme, schedule included), `dark` or
+- **Dark or bright.** `Themes.mode` is `auto` (a light sensor where the device allows one, `theme/light.dart`;
+  else Android's dark theme, schedule included), `dark` or
   `bright`, chosen in Settings and kept across restarts. The status bar's button holds the other variant;
   a long press there is auto again.
 - **Cosmetic fonts are opt-in.** A widget that wants the HUD face names `fontFamily: hudFont` in its own