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/bar/Workspaces.qml

@@ -36,7 +36,7 @@ Item {
 
     implicitWidth: row.implicitWidth + 12
     // Full bar height makes (0, 0) part of the wheel target. The dots stay
-    // optically centred by the row and active-pill geometry below.
+    // optically centered by the row and active-pill geometry below.
     implicitHeight: Theme.barHeight
 
     // Scroll over the padding either side of the row, not just over a dot.
@@ -59,7 +59,7 @@ Item {
         border.width: 0 // QTBUG-137166: rounded rects can lose their corners
 
         // The prism, as a fill rather than a hairline. This is the one element
-        // in the bar that is always coloured, so it is where the pair belongs.
+        // in the bar that is always colored, so it is where the pair belongs.
         gradient: Gradient {
             orientation: Gradient.Horizontal
             GradientStop {