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/DesktopPreview.qml

@@ -50,7 +50,7 @@ Rectangle {
 
     // Stands in for the wallpaper. Static: an animated gradient here would
     // repaint forever behind a settings page.
-    // Follows the colour scheme. A preview that stays dark while the shell
+    // Follows the color scheme. A preview that stays dark while the shell
     // around it is light does not read as "your desktop" -- it reads as a
     // screenshot of someone else's.
     gradient: Gradient {
@@ -129,7 +129,7 @@ Rectangle {
         required property bool focused
 
         // The gradient border is drawn as a filled rounded rect with the window
-        // body inset on top of it: QML's Rectangle border takes a colour, not a
+        // body inset on top of it: QML's Rectangle border takes a color, not a
         // gradient, and the prism border is the whole signature here.
         Rectangle {
             id: frame