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/hypr/rules.lua

@@ -69,7 +69,7 @@ hl.window_rule({
     float = true,
 })
 
--- Settings-style utility windows and pickers: float and centre, like GNOME did.
+-- Settings-style utility windows and pickers: float and center, like GNOME did.
 hl.window_rule({
     match = { class = "^(pavucontrol|org\\.pulseaudio\\.pavucontrol|nm-connection-editor|blueman-manager|org\\.gnome\\.Settings)$" },
     float  = true,
@@ -150,7 +150,7 @@ hl.layer_rule({
     ignore_alpha = 0.3,
 })
 
--- Popovers: calendar, quick settings, notification centre, tray menus.
+-- Popovers: calendar, quick settings, notification center, tray menus.
 hl.layer_rule({
     name  = "qs-popover",
     match = { namespace = "^qs-popover" },