Remember the brightness level: persist it and re-apply on slideshow start and foreground

[kipp-ing]

Decided: 2026-09-13 product-facts pass (decision D-08, fact UNATT-02). A dimmed frame is a supported use and may be advertised; this makes it hold.

Today: the in-app brightness level is not saved. It's lost on relaunch and when leaving the slideshow, and not re-applied when the app comes back to the foreground (for example after lock/unlock). Spec 400 never required persistence, so this is a feature gap, not a regression.

Todo

• Spec: add an FR to specs/400-power-manager/spec.md (persist the level; re-apply on slideshow start and on foreground; keep restoring the system baseline on exit).
• TDD: red test in PowerKit for persist + re-apply, then implement.

[kipp-ing]

Deferred (Jan, 2026-09-14): this becomes its own feature spec, not a fix inside 400.

While specifying it (#69 pass), the question turned out to be which level to remember. The in-app slider, Home Assistant and Shortcuts can all set brightness, and a night automation that sets 0% must not bring the frame up near black after a morning relaunch. Jan: a new feature with a new spec covering HA, the ambient light sensor / automatic brightness and night time together.

Carried into the future spec (recorded in specs/400-power-manager/spec.md → Roadmap):

• Re-applying on foreground must re-capture the brightness baseline for each foreground session (FR-400-10/11, SC-400-05); otherwise exit restores a stale value.
• The HA light reports a hard-coded 1.0 at start (SlideshowRemoteControlAdapter initialBrightness) instead of the applied level.
• Relation to the reserved presence sleep/wake sub-spec 730.

Leaving this issue open as the pointer.