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/quicksettings/BrightnessControl.qml

@@ -29,7 +29,7 @@ Item {
     property real backlightValue: 0
 
     readonly property int rowCount: (root.backlightAvailable ? 1 : 0) + Brightness.displays.length
-    readonly property bool labelled: root.rowCount > 1
+    readonly property bool labeled: root.rowCount > 1
 
     visible: root.rowCount > 0
     implicitHeight: rows.implicitHeight
@@ -89,7 +89,7 @@ Item {
                 // Hyprland already knows what each output is called, so the
                 // name comes from there rather than from a second source that
                 // could disagree with the Displays page. The connector is the
-                // fallback, so a display is never an unlabelled slider.
+                // fallback, so a display is never an unlabeled slider.
                 label: Displays.monitorNamed(modelData.connector)?.description || modelData.connector
                 value: modelData.value / 100
                 onMoved: v => Brightness.set(modelData.bus, Math.round(v * 100))
@@ -113,7 +113,7 @@ Item {
             anchors.leftMargin: 6
             anchors.rightMargin: 6
             anchors.top: parent.top
-            visible: root.labelled
+            visible: root.labeled
             height: visible ? implicitHeight + 2 : 0
             text: row.label
             color: Theme.fgDim
@@ -124,7 +124,7 @@ Item {
 
         // The slider and its glyph share one strip so the two stay aligned
         // whether or not a caption sits above them. Anchoring the glyph to
-        // both a caption and a centre line instead would conflict, and an
+        // both a caption and a center line instead would conflict, and an
         // anchor set to undefined is not released.
         Item {
             id: control
@@ -134,7 +134,7 @@ Item {
             height: 32
 
             // ValueSlider draws its own leading icon, but symbolic icons need
-            // recolouring to be visible — see ThemedIcon.
+            // recoloring to be visible — see ThemedIcon.
             ThemedIcon {
                 id: glyph
                 anchors.left: parent.left