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/datemenu/DateMenu.qml

@@ -1,12 +1,12 @@
 // GNOME's date/time menu: calendar, ongoing activity and messages in one panel,
-// dropping from the top centre because that is where GNOME put it and the
+// dropping from the top center because that is where GNOME put it and the
 // muscle memory is the whole point.
 //
 // A layer-shell surface rather than a PopupWindow because the inline reply
 // fields have to be typeable, and because the clock it hangs from is owned by a
 // different module.
 //
-// Layer-shell centres a surface on whichever axis it is not anchored to, so
+// Layer-shell centers a surface on whichever axis it is not anchored to, so
 // anchoring only `top` is what puts it in the middle.
 
 import QtQuick
@@ -87,7 +87,7 @@ PanelWindow {
         }
 
         // Grows out of the top edge it is anchored to, rather than out of its
-        // own centre — the panel should look like it came from the bar.
+        // own center — the panel should look like it came from the bar.
         transform: Scale {
             id: openScale
             origin.x: surface.width / 2