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/ValueSlider.qml

@@ -25,7 +25,7 @@ Item {
     opacity: enabled ? 1.0 : 0.45
 
     // ThemedIcon, not a bare IconImage — see the note in Toggle.qml: Adwaita
-    // symbolic icons have a hardcoded near-black fill that Qt will not recolour.
+    // symbolic icons have a hardcoded near-black fill that Qt will not recolor.
     ThemedIcon {
         id: leadingIcon
         anchors.left: parent.left
@@ -57,7 +57,7 @@ Item {
             border.width: 0
 
             // The gradient is anchored to the full track, not to the fill, so
-            // the colour at a given position never moves as the value changes —
+            // the color at a given position never moves as the value changes —
             // dragging reveals the prism rather than squashing it.
             gradient: Gradient {
                 orientation: Gradient.Horizontal