quickshell: snap kbd backlight keys to exact 5% steps

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

settings/quickshell.nix

@@ -1645,13 +1645,31 @@ in
                     brightProc.running = true;
                 }
 
+                // smc::kbd_backlight has a raw max of 255, so brightnessctl's
+                // own percentages don't round-trip: `10%+` adds 25 raw, which
+                // reads back as 9%. Snap to exact 5% steps in raw units and
+                // report the intended percent to the OSD instead of the
+                // truncated readback. Prints a fake -m line so the parser above
+                // is unchanged. $3 = current raw, $5 = max raw.
+                function setKbd(step) {
+                    if (brightProc.running) return;
+                    brightProc.osdIcon = "keyboard";
+                    brightProc.command = ["sh", "-c",
+                          'd=smc::kbd_backlight; set -- $(brightnessctl -m -d $d | tr , " "); '
+                        + 'n=$(( ($3 * 20 + $5 / 2) / $5 ' + step + ' )); '
+                        + '[ $n -lt 0 ] && n=0; [ $n -gt 20 ] && n=20; '
+                        + 'brightnessctl -q -d $d set $(( (n * $5 + 10) / 20 )); '
+                        + 'echo "$d,leds,0,$(( n * 5 ))%,$5"'];
+                    brightProc.running = true;
+                }
+
                 IpcHandler {
                     target: "osd"
                     // floor 1%: `set 0%` turns the panel fully off
                     function brightnessUp(): void   { root.setBrightness(["set", "5%+"], "brightness"); }
                     function brightnessDown(): void { root.setBrightness(["-n", "1", "set", "5%-"], "brightness"); }
-                    function kbdUp(): void   { root.setBrightness(["-d", "smc::kbd_backlight", "set", "10%+"], "keyboard"); }
+                    function kbdUp(): void   { root.setKbd("+ 1"); }
-                    function kbdDown(): void { root.setBrightness(["-d", "smc::kbd_backlight", "set", "10%-"], "keyboard"); }
+                    function kbdDown(): void { root.setKbd("- 1"); }
                 }
 
                 // ── Night light: quickshell owns the schedule, the hyprsunset