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/AccentPicker.qml

@@ -2,10 +2,10 @@
 //
 // Each swatch is drawn as the GRADIENT it will actually produce, not a flat
 // dot, because the gradient is the thing being chosen -- the focused window
-// border, the bar hairline and every active state are the two colours meeting.
+// border, the bar hairline and every active state are the two colors meeting.
 // A row of flat circles would misrepresent all of them.
 //
-// Named accents rather than a colour wheel: each name carries a curated pair
+// Named accents rather than a color wheel: each name carries a curated pair
 // per scheme, so every choice stays legible in both light and dark. See
 // config/Theme.qml for the palette and the reasoning.
 
@@ -46,7 +46,7 @@ Flow {
 
             // The hit target is the whole swatch+label unit, not just the
             // 46px circle: the label exists specifically so someone with a
-            // colour vision deficiency can identify an accent without it, and
+            // color vision deficiency can identify an accent without it, and
             // a label that cannot itself be tapped defeats that.
             HoverHandler {
                 cursorShape: Qt.PointingHandCursor
@@ -66,7 +66,7 @@ Flow {
                 anchors.horizontalCenter: parent.horizontalCenter
                 color: "transparent"
                 // The ring sits outside the gradient rather than over it, so a
-                // selected swatch still shows its true colours.
+                // selected swatch still shows its true colors.
                 border.width: entry.selected ? 2 : 1
                 border.color: entry.selected ? Theme.fg : Theme.alpha(Theme.fg, 0.14)
 
@@ -83,8 +83,8 @@ Flow {
                 }
             }
 
-            // Always shown, not a tooltip. Telling swatches apart by colour is
-            // exactly what someone with a colour vision deficiency cannot do,
+            // Always shown, not a tooltip. Telling swatches apart by color is
+            // exactly what someone with a color vision deficiency cannot do,
             // and it is the reason the palette is named rather than freeform --
             // hiding the names behind a hover would waste that.
             Text {