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/capture/CaptureBar.qml

@@ -1,7 +1,7 @@
 // The floating control panel at the bottom of the capture overlay.
 //
 // Layout is a deliberate copy of GNOME 42's screenshot UI: the area segmented
-// control on top, and below it the shutter dead-centre with "Show Pointer" on
+// control on top, and below it the shutter dead-center with "Show Pointer" on
 // the left and the screenshot/screencast switch on the right.
 
 import QtQuick
@@ -13,7 +13,7 @@ Rectangle {
 
     signal captureRequested
 
-    // Set by the overlay so the shutter can grey out when Window mode has
+    // Set by the overlay so the shutter can gray out when Window mode has
     // nothing pickable (no Hyprland, or an empty workspace).
     property bool captureEnabled: true