sessiond: dim by percent, not an absolute

brightnessctl set 10 is 4% on blueline's 255-step backlight and 0.5% on
the iPhone 7's 2047-step Apple DWI, where it reads as fully off. Measured
on d10 2026-08-09: brightness=1 with the panel still powered, so the
screen went black on a dim that never blanked.

src/sessiond/server.rs

@@ -361,7 +361,10 @@ fn execute(shared: &Arc<Shared>, action: Action) {
         // sessiond guarantees this runs once per dim, which is what the old
         // hypridle `-s`/`-r` listener could not: a second save while already
         // dim is what pinned brightness at 10/255.
-        Action::Dim => ("brightnessctl", vec!["set", "10"], "panel-dim"),
+        // Percent, not an absolute. `set 10` is 10/255 on blueline and 10/2047
+        // on the iPhone 7's Apple DWI backlight, where it reads as fully off —
+        // measured 2026-08-09, brightness=1 with the panel still powered.
+        Action::Dim => ("brightnessctl", vec!["set", "4%"], "panel-dim"),
         Action::Restore => unreachable!("handled above; the restore carries a value"),
         // The other direction, through the same executor.
         //