sway: center wallpaper, fix lockscreen flags and add idle timeouts

- Use bg mode 'center' so the wallpaper keeps its native size.
- Drop --clock and --indicator from the lockscreen invocation; those
  flags belong to swaylock-effects and made vanilla swaylock exit with
  an unknown-option error. Keep --indicator-idle-visible to preserve
  the ring while idle.
- Replace the suspend-only swayidle line with a full idle policy:
  5 min lock, 10 min DPMS off, DPMS on on resume, lock before sleep.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dotfiles/desktop/.config/sway/scripts/lockscreen

@@ -3,8 +3,7 @@
 exec swaylock \
     --image "$HOME/.config/sway/wallpapers/wallpaper-161664.jpg" \
     --scaling fill \
-    --clock \
-    --indicator \
+    --indicator-idle-visible \
     --ring-color 2f4058 \
     --inside-color 1f1f28 \
     --line-color 00000000 \