Add Software Update, across packages, applications and firmware

Three sources that fail independently, so they are counted and applied
separately: a flatpak mirror being down says nothing about whether a
kernel security fix is waiting. Blending them into one number would hide
exactly the case that matters.

Checking costs about nine seconds, which is too long to spend every time
a page opens, so the page opens on the last result and says when it was
taken. A first visit with nothing cached goes and finds out rather than
showing a confident "up to date" it has no basis for.

Installing packages takes a snapshot first, named after what is about to
happen, so Snapshots shows "before 32 package updates" rather than a
timestamp. Best effort: a machine without snapper still updates, because
an update that refuses to run when a nicety fails would be worse than
one without a restore point.

Automatic updates cover applications only, through a Panama-owned user
timer running daily with a randomized delay. Packages still ask, and
dnf-automatic is reported as absent rather than offered, because
installing software is not a settings action.

Health gained a check, and that is where the bug was: it first returned
status "degraded", which is not in the doctor's vocabulary of ok,
warning, error and unconfigured. It was counted as nothing at all while
the summary still said healthy -- the same silent no-op this codebase
keeps relearning. A contract now asserts every status a check can return
is one the doctor counts, and the doctor's own contract knows about the
new check rather than failing on its arrival.

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

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

@@ -42,6 +42,7 @@ Rectangle {
         { page: "power", label: "Power & Lock", icon: "\u{F0425}" },
         { page: "datetime", label: "Date & Time", icon: "\u{F0954}" },
         { page: "applications", label: "Applications", icon: "\u{F003B}" },
+        { page: "updates", label: "Software Update", icon: "\u{F06B0}" },
         { page: "storage", label: "Storage", icon: "\u{F02CA}" },
         { page: "snapshots", label: "Snapshots", icon: "\u{F0954}" },
         { page: "users", label: "Users", icon: "\u{F0004}" },