settings: wallpaper download + purity toggle, repo-owned

Adds a Download-random-wallpaper button and a 3-way content filter
(SFW/Sketchy/NSFW) to our Wallpaper settings page, backed by a new
WallpaperDownload service and its own download script under
surfaces/quickshell/scripts/wallpaper/ — not ii's scripts/colors tree,
which is going away. Only the 'wallpapers apply' IPC target still comes
from ii and moves with it when the base is vendored.

Downloads are named by wallhaven id so the library accumulates instead of
overwriting a single wallhaven_wallpaper.<ext>. Purity is read from and
written to background.wallhaven.purity in config.json (not in the upstream
Config.options schema). Anchors the root scripts/ gitignore to / so shipped
surface scripts are tracked.

.gitignore

@@ -67,8 +67,10 @@ AD_continuation_prompt.md
 COMMIT_MESSAGES.txt
 vanguard-souveraine-notes.md
 
-# Local-only project scaffolding — kept on disk, never committed to any remote
-.superpowers/
-journal/
-scripts/
-website/
+# Local-only project scaffolding — kept on disk, never committed to any remote.
+# Anchored to repo root (leading /) so shipped surface scripts — e.g.
+# surfaces/quickshell/scripts/ — are still tracked.
+/.superpowers/
+/journal/
+/scripts/
+/website/