v4.9 - uniform overlay fill + a clearer dashboard

A focused follow-up to 4.8: bright static screens get a cleaner, more even dim, the dashboard explains itself in plain language, and the overlay now covers every pixel of the screen width.

• Even protection on bright static screens - large bright pages and documents now dim as one smooth, uniform sheet instead of a patchy, blotchy overlay. The change is display-only; the underlying burn-in tracking is unchanged.

• The dashboard explains itself in plain language - the classifier's reasoning now shows as simple tags like Media, Idle, Fullscreen, Video, or Controller, instead of a dense technical line.

• Full-width overlay coverage - the thin uncovered line at the screen edge is gone; the overlay now covers the entire width while still avoiding Windows' fullscreen mode, so Focus Assist and the taskbar behave normally.