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/PowerProfiles.qml

@@ -33,7 +33,7 @@ Singleton {
 
     // Non-empty when the machine cannot actually deliver the profile it is set
     // to -- thermal throttling, or a laptop running on battery. Worth showing,
-    // because otherwise "performance" is a claim the hardware is not honouring.
+    // because otherwise "performance" is a claim the hardware is not honoring.
     property string degraded: ""
 
     readonly property bool available: root.profiles.length > 0
@@ -52,7 +52,7 @@ Singleton {
     function detail(profile: string): string {
         switch (profile) {
         case "power-saver": return "Reduces performance to save energy and run quieter";
-        case "balanced": return "Standard behaviour, scaling up only when needed";
+        case "balanced": return "Standard behavior, scaling up only when needed";
         case "performance": return "Holds higher clocks, using more power and making more noise";
         default: return "";
         }