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/SettingsBackup.qml

@@ -107,7 +107,7 @@ Singleton {
                 if (restoreAccepted)
                     root.lastError = "";
                 else if (root.lastError === "")
-                    root.lastError = "Desktop settings were restored, but Home favourites could not be reloaded.";
+                    root.lastError = "Desktop settings were restored, but Home favorites could not be reloaded.";
                 if (!restoreAccepted) {
                     root.setDisplayBlocked(false);
                     root.protectedDisplays = ({});
@@ -192,10 +192,10 @@ Singleton {
         if (actionRun.running)
             return;
         actionRun.restoring = false;
-        actionRun.exec([root.helperPath, "save", root.serialiseHomeState()]);
+        actionRun.exec([root.helperPath, "save", root.serializeHomeState()]);
     }
 
-    function serialiseHomeState(): string {
+    function serializeHomeState(): string {
         const current = root.readHomeState();
         const favorites = [];
         for (const favorite of current.favorites ?? []) {