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/capture/SelectionLayer.qml

@@ -1,6 +1,6 @@
 // Selection mode: a GNOME-style rubber band over the frozen screen.
 //
-// Behaviours copied from GNOME 42: the overlay opens with a sensible centred
+// Behaviors copied from GNOME 42: the overlay opens with a sensible centered
 // selection already in place (so the shutter is immediately useful), dragging
 // inside moves it, dragging a corner resizes from the opposite corner, and
 // dragging anywhere else starts a fresh band.
@@ -52,7 +52,7 @@ Item {
             root.resetSelection();
     }
 
-    // Centred, roughly the middle half of the screen.
+    // Centered, roughly the middle half of the screen.
     function resetSelection(): void {
         if (root.width <= 0 || root.height <= 0)
             return;