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/settings/DateTimePage.qml

@@ -7,7 +7,7 @@
 // because that genuinely is a Panama preference.
 //
 // Changing the timezone or network time needs privilege. timedatectl asks
-// polkit, and a cancelled dialog surfaces as an error rather than as a value
+// polkit, and a canceled dialog surfaces as an error rather than as a value
 // that appears to have been accepted.
 
 import QtQuick
@@ -33,7 +33,7 @@ SettingsPage {
         SettingRow {
             label: "Set automatically"
             detail: DateTime.ntpEnabled
-                ? (DateTime.ntpSynchronised ? "Synchronised with a time server" : "Waiting to synchronise")
+                ? (DateTime.ntpSynchronized ? "Synchronized with a time server" : "Waiting to synchronize")
                 : "The clock is set by hand"
             controlWidth: 48
             divider: false