Add a Storage page

Nothing showed what was using the drive, and removable media was handled
by a tray helper with no surface in Settings at all.

One scroll rather than tabs: space above, the device below. Every other
settings page is a scrolling card stack, and a tab would not be
deep-linkable from the launcher command or from search.

Three things the page has to get right, each now pinned by a contract,
because each is a way it could quietly lie. / and /home are one btrfs
filesystem sharing one pool of free space, and a page that copies df
shows double the free space that exists. zram is a block device and is
not storage; counting it as a drive overstates this machine by 8 GB.
Unmount and eject refuse anything not on a removable drive, because the
UI is what asks and a UI can be wrong.

The cheap read -- layout, usage, health -- runs when the page opens, at
around 90ms. Measuring what is filling the drive means walking every
file, so it happens on request and says so rather than showing an empty
list that reads as "nothing here".

Partitioning and formatting are deliberately absent. A settings pane is
the wrong place to put erasing a disk two clicks deep; the page opens
GNOME Disks for that.

Adding the page found a fourth hard-coded page list in ShellState. A
page missing from it does not error -- openSettings() falls back to
"home", so the launcher opens the wrong page and logs nothing. A
registry contract now holds the three lists together.

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

config/dot/quickshell/services/SettingsSearch.qml

@@ -64,6 +64,11 @@ Singleton {
         { label: "Bluetooth", detail: "Managed by GNOME Settings", page: "connectivity" },
         { label: "Printers", detail: "Managed by GNOME Settings", page: "connectivity" },
         { label: "Default applications", detail: "Browser, mail, files", page: "applications" },
+        { label: "Free space", detail: "How full each drive and filesystem is", page: "storage" },
+        { label: "Disk usage", detail: "What is using the space on this machine", page: "storage" },
+        { label: "Drive health", detail: "Temperature, hours powered on, and reported warnings", page: "storage" },
+        { label: "Removable drives", detail: "Unmount a USB drive or memory card safely", page: "storage" },
+        { label: "Encryption", detail: "Whether the filesystem is encrypted", page: "storage" },
         { label: "Output volume", detail: "Choose the output device and its level", page: "sound" },
         { label: "Input volume", detail: "Choose the microphone and its level", page: "sound" },
         { label: "Per-application volume", detail: "Set the level of each application separately", page: "sound" },