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

tests/quickshell/enum-hypr-map-contract.sh

@@ -2,9 +2,9 @@
 
 # Every enum backed by a Hyprland option must offer values that option accepts.
 #
-# This exists because of a bug that shipped: followMouse offered 0/1/2 labelled
+# This exists because of a bug that shipped: followMouse offered 0/1/2 labeled
 # "Never" / "Click to focus" / "Sloppy focus", while Hyprland's actual mapping
-# is disabled=0, follow=1, detached=2, separate=3. The desktop was labelled
+# is disabled=0, follow=1, detached=2, separate=3. The desktop was labeled
 # "Click to focus" and was in fact following the pointer, the way to GET click
 # to focus was to choose "Never", and value 3 did not exist in the UI at all.
 #
@@ -85,7 +85,7 @@ while IFS=$'\t' read -r key option values; do
         while read -r published; do
             [[ -n "$published" ]] || continue
             grep -qx "$published" <<<"$(tr ',' '\n' <<<"$values")" \
-                || fail "$option publishes value $published but $key does not offer it, so that behaviour is unreachable from Settings"
+                || fail "$option publishes value $published but $key does not offer it, so that behavior is unreachable from Settings"
         done <<<"$(tr ',' '\n' <<<"$map_values")"
     fi