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/ShortcutCapture.qml

@@ -19,7 +19,7 @@ FocusScope {
     id: root
 
     signal captured(string chord)
-    signal cancelled
+    signal canceled
 
     property string message: ""
 
@@ -97,7 +97,7 @@ FocusScope {
         event.accepted = true;
 
         if (event.key === 0x01000000) {   // Escape
-            root.cancelled();
+            root.canceled();
             return;
         }
         if (root.isModifierOnly(event.key))
@@ -127,6 +127,6 @@ FocusScope {
 
     onActiveFocusChanged: {
         if (!activeFocus)
-            root.cancelled();
+            root.canceled();
     }
 }