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/bar/Clock.qml

@@ -1,4 +1,4 @@
-// The centre clock, kept in the same place and the same format GNOME had it:
+// The center clock, kept in the same place and the same format GNOME had it:
 // weekday, date, 12-hour time with seconds. Clicking it opens the calendar,
 // exactly like GNOME's date menu.
 
@@ -35,7 +35,7 @@ Pill {
         anchors.verticalCenter: parent.verticalCenter
         text: Qt.formatDateTime(clock.date, root.format)
         font.family: Theme.fontFamily
-        // Seconds tick once a second in the centre of the bar; without tabular
+        // Seconds tick once a second in the center of the bar; without tabular
         // figures the whole clock shifts sideways on every digit change.
         font.features: Theme.tabularFigures
         font.pixelSize: Theme.fontSize