Convert British spellings to American across the repo

colour -> color, behaviour -> behavior, centre -> center, favourite ->
favorite, and about twenty other pairs, applied consistently across
comments, docs, error/UI copy, and a handful of QML identifiers that
used the British spelling as their actual name: SystemSettings'
serialiseValue/serialiseTable/normaliseGradient, Displays'
normaliseModes, Wallpaper's normalisePolicy, SettingsBackup's
serialiseHomeState, DateTime's ntpSynchronised property, Clipboard's
_normalise helper, and ShortcutCapture's cancelled signal (with its
onCancelled handler in ShortcutsPage.qml). Every call site and the two
tests that assert on the literal source text (settings-ownership and
settings-backup-live contracts) were updated in lockstep.

Left untouched: config/dot/espanso/match/packages/misspell-en/ is a
vendored third-party autocorrect dictionary -- its entries are typo
corrections, not our prose, and rewriting them would fight the
package's own purpose (and any future re-sync from upstream).

The already-American `favorites` property (Home page pinned
accessories) was never actually misspelled -- only nearby comments and
error strings said "favourites" -- so no data migration was needed
there.

Claude-Session: https://claude.ai/code/session_01E6TJUAh41HaP25MVHWkhRZ

config/dot/quickshell/services/Notifs.qml

@@ -10,7 +10,7 @@ pragma Singleton
 // the *live* objects rather than copies: that keeps actions and inline replies
 // working from the tray, which is what GNOME does. The cost is that a
 // notification an app closes itself (progress bars, "download finished"
-// replacing "downloading") disappears from history too — correct behaviour,
+// replacing "downloading") disappears from history too — correct behavior,
 // but the reason history is not append-only.
 // ─────────────────────────────────────────────────────────────────────────────
 
@@ -34,7 +34,7 @@ Singleton {
     // Suppresses toasts entirely. Notifications still reach history.
     property bool doNotDisturb: false
 
-    // Cleared when the notification centre is opened. The bar binds to this.
+    // Cleared when the notification center is opened. The bar binds to this.
     property int unreadCount: 0
 
     // Kept separate from the persisted map so this version can safely run