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/services/DateTime.qml

@@ -23,7 +23,7 @@ Singleton {
 
     property string timezone: ""
     property bool ntpEnabled: false
-    property bool ntpSynchronised: false
+    property bool ntpSynchronized: false
     property string localTime: ""
     property string universalTime: ""
     property string rtcTime: ""
@@ -79,7 +79,7 @@ Singleton {
             // status is re-read either way.
             root.lastError = exitCode === 0
                 ? ""
-                : "The system rejected that change, or authentication was cancelled.";
+                : "The system rejected that change, or authentication was canceled.";
             root.refresh();
         }
     }
@@ -101,7 +101,7 @@ Singleton {
             else if (key === "NTP")
                 root.ntpEnabled = value === "yes";
             else if (key === "NTPSynchronized")
-                root.ntpSynchronised = value === "yes";
+                root.ntpSynchronized = value === "yes";
         }
         root.lastError = "";
     }
@@ -115,7 +115,7 @@ Singleton {
     // caller-supplied text reaches the command.
     function setTimezone(zone: string): bool {
         if (root.zones.indexOf(zone) < 0) {
-            root.lastError = "That is not a timezone this system recognises.";
+            root.lastError = "That is not a timezone this system recognizes.";
             return false;
         }
         if (writeRun.running)