Save things where the rest of the software already saves them

Screenshots and recordings offered three folders to choose between, and three
guesses cannot include the folder somebody's other software already writes to --
which is the only folder that matters. This machine has had ~/Pictures/Screenshots
and ~/Videos/Screencasts since long before Panama, and Panama was writing
recordings to a Videos/Recordings it invented. Both are free text now, and the
recording default is the folder that was already there.

Wallpapers were swept from four directories at once, so the distribution's stock
images arrived mixed in with the user's own and there was no way to ask for just
one. Where wallpapers live is something somebody knows about their own machine.
It is a setting, not a search.

All three accept an absolute path as well as one relative to home, which meant
fixing Capture: it prefixed $HOME unconditionally, so naming /mnt/captures would
have written screenshots to ~/mnt/captures and left nobody able to find them.

The generator turned out to skip any entry whose comment sits inside the braces
rather than above them -- it looks for `key:` immediately after `{`. Three
settings were invisible in the reference because of it, one of them dockScreens,
which has never appeared there at all. The staleness contract could not see it
either: regenerating reproduced the same omission, so the copy was current and
incomplete at once. It now counts what was declared against what it could read
and refuses rather than quietly documenting less than exists.

Claude-Session: https://claude.ai/code/session_01Q84axqUE5inJhf5Jz9CFy1

config/dot/quickshell/modules/settings/AppearancePage.qml

@@ -74,7 +74,9 @@ SettingsPage {
         title: "Background"
         subtitle: Wallpaper.lastError !== ""
             ? Wallpaper.lastError
-            : "Applied to every display. Looked for in ~/Pictures/Wallpapers, ~/Pictures/Backgrounds, ~/.local/share/backgrounds, and /usr/share/backgrounds."
+            : "Applied to every display, from the folder below."
+
+        TextEntryRow { setting: "wallpaperDir"; placeholder: "Pictures/Wallpapers" }
 
         WallpaperControls {
             id: wallpaperControls