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/Toggle.qml

@@ -1,5 +1,5 @@
 // A GNOME-style quick-settings tile: big rounded button with an icon and a
-// label, filled with the accent colour when active.
+// label, filled with the accent color when active.
 
 import QtQuick
 import Quickshell
@@ -52,7 +52,7 @@ Rectangle {
         spacing: 10
 
         // ThemedIcon, not a bare IconImage: Adwaita's symbolic SVGs carry a
-        // hardcoded #2e3436 fill and Qt does not recolour them the way GTK
+        // hardcoded #2e3436 fill and Qt does not recolor them the way GTK
         // does, so drawn directly they are all but invisible on this palette.
         ThemedIcon {
             anchors.verticalCenter: parent.verticalCenter