Close the sweep's last blind spot, and stop shortcuts silently colliding

gapsIn and gapsOut were the only two compositor settings the write sweep had
never verified: Hyprland answers for them in CSS shorthand, "5 5 5 5", and the
sweep had no way to compare that. The preference behind each is a single int
that Hyprland expands to four sides, so a uniform reading compares exactly. A
non-uniform one is not something the preference can express, and is skipped
rather than collapsed to a number it never wrote. 63 of 63 verified live now,
none skipped.

Wallpaper thumbnails are cached. The report that five of them sat at "Loading…"
was a screenshot taken 1.1 seconds after the page opened -- decoding one of
these at tile size takes between 1.2 and 2.6 seconds and about ten start at
once, which the code already said. Measuring it did turn up something real
though: without a cache, scrolling back up pays that decode again for every
tile. The tradeoff is a wallpaper replaced in place showing a stale thumbnail
until restart, which is worth it for a directory of files that are added rather
than edited.

A chord already in use is now named rather than taken: "Super+Q is already
Terminal". Two actions on one chord means whichever Hyprland reads last wins,
which is not a thing to find out later by pressing it. Rebinding a shortcut to
the chord it already holds is correctly not a conflict.

Also: Open Appearance lands on the Windows tab now that the page has tabs,
Storage points at reclaimable container space, and a dock row shows its desktop
id only when two pinned applications share a name -- it is developer text, and
repeating it under fifteen recognisable names made the list harder to scan.

Written down because it cost the shell: QML has no default parameter values, and
`function openSettings(page: string, section: string = "")` fails the entire
configuration rather than the one function -- so the bar and dock went with it,
and 43 contracts failed at once pointing at the same line. qmllint --bare passes
that, which is why the usual check before touching the running shell did not
catch it. openSettingsSection exists as a separate function for that reason.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

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

@@ -278,6 +278,25 @@ SettingsPage {
         }
     }
 
+    // Named here because this is the page somebody opens when space is short,
+    // and container images are usually the largest thing nobody remembers
+    // having. Only shown when there is actually something to reclaim.
+    SettingsCard {
+        visible: Containers.available && Containers.reclaimable > 0
+        title: "Containers are holding " + Containers.formatBytes(Containers.reclaimable)
+
+        ActionRow {
+            label: "Images and volumes nothing references"
+            detail: Containers.unusedImages.length + " image"
+                + (Containers.unusedImages.length === 1 ? "" : "s") + " and "
+                + Containers.unusedVolumes.length + " volume"
+                + (Containers.unusedVolumes.length === 1 ? "" : "s")
+            action: "Review"
+            divider: false
+            onTriggered: ShellState.settingsPage = "containers"
+        }
+    }
+
     SettingsCard {
         title: "Removable drives"
         subtitle: "USB drives and memory cards."