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/modules/notifications/NotificationCard.qml

@@ -1,7 +1,7 @@
 // One notification, rendered GNOME-style: app icon, app name, summary, body,
 // optional image, action buttons and an inline reply box.
 //
-// Shared by the toasts and the notification centre so a notification looks the
+// Shared by the toasts and the notification center so a notification looks the
 // same wherever you meet it.
 //
 // IconButton/ThemedIcon come from the quicksettings module — they belong in
@@ -21,7 +21,7 @@ Rectangle {
 
     required property var notification
 
-    // The centre renders a denser card than a toast banner.
+    // The center renders a denser card than a toast banner.
     property bool compact: false
 
     signal dismissed
@@ -43,7 +43,7 @@ Rectangle {
         }
     }
 
-    // Critical notifications get a red edge rather than a colour wash, so the
+    // Critical notifications get a red edge rather than a color wash, so the
     // text contrast never changes.
     Rectangle {
         anchors.left: parent.left