Fold thirty-one settings pages into fifteen categories with tabs

The sidebar was a flat scan of thirty-one rows; now it reads like a
settings app. Multi-subject categories (Input, Network & Sharing,
Applications, Users & Accounts, Privacy & Security, System) carry an
Appearance-style tab strip above the page, drawn by the shell so the
leaf pages themselves are untouched. The taxonomy lives in one new
file, services/SettingsRoutes.qml; the sidebar, the strip, route
validation, search breadcrumbs, and both generators derive from it.

ShellState.settingsPage still holds leaf ids, so every deep link, IPC
call, and search result keeps working — and now lands on the exact
tab. Dictation moves out of Sound onto its own page under Input, with
a handoff back to Sound for the microphone. The strip scrolls when
System's nine tabs outgrow a tiled window. All 161 contracts pass.

Claude-Session: https://claude.ai/code/session_01Ms2FbjQy31TVf3CEvQhGM8

config/dot/quickshell/manual/02-the-keyboard.md

@@ -55,6 +55,6 @@ one I have".
 
 ## Changing them
 
-Settings has a Shortcuts page listing every bind, each of which can be
-reassigned. A rebind moves the shortcut and cannot change what it does, so
+Settings has a Keyboard page, under Input, listing every bind, each of which
+can be reassigned. A rebind moves the shortcut and cannot change what it does, so
 there is no way to make a key do something unexpected by editing it.