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/widgets/PrismEdge.qml

@@ -24,7 +24,7 @@ Rectangle {
     id: root
 
     // Horizontal inset, normally the parent's corner radius. Below that the
-    // line runs out onto the curve and looks like a rendering artefact.
+    // line runs out onto the curve and looks like a rendering artifact.
     property real inset: Theme.popoverRadius
 
     anchors.leftMargin: root.inset
@@ -33,7 +33,7 @@ Rectangle {
     implicitHeight: 1
     height: 1
 
-    // The colour stops are the only place the two accents touch.
+    // The color stops are the only place the two accents touch.
     gradient: Gradient {
         orientation: Gradient.Horizontal