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/Wallpaper.qml

@@ -187,7 +187,7 @@ Singleton {
         };
     }
 
-    function normalisePolicy(policy: var): var {
+    function normalizePolicy(policy: var): var {
         if (!policy || typeof policy !== "object")
             return null;
         const mode = ["single", "slideshow", "per-monitor"].includes(policy.mode)
@@ -220,8 +220,8 @@ Singleton {
     function applyPolicy(policy: var, persist: bool, automatic: bool): bool {
         if (root.busy)
             return false;
-        const normalised = root.normalisePolicy(policy);
-        if (normalised === null) {
+        const normalized = root.normalizePolicy(policy);
+        if (normalized === null) {
             root.lastError = "That wallpaper policy is not valid.";
             return false;
         }
@@ -231,8 +231,8 @@ Singleton {
             return false;
         }
         const expected = WallpaperPolicy.effectiveMap(
-            normalised.mode, normalised.globalPath, normalised.slideshowPath,
-            normalised.assignments, outputs, normalised.candidates);
+            normalized.mode, normalized.globalPath, normalized.slideshowPath,
+            normalized.assignments, outputs, normalized.candidates);
         if (Object.keys(expected).length !== outputs.length
                 || Object.values(expected).some(path => path === "")) {
             root.lastError = "That wallpaper policy is not valid.";
@@ -242,7 +242,7 @@ Singleton {
         root.transaction = {
             expected,
             remaining: outputs.slice(),
-            policy: normalised,
+            policy: normalized,
             persist: persist === true,
             automatic: automatic === true
         };