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

tests/quickshell/settings-commit-reset-contract.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-# Two behaviours the settings rows depend on:
+# Two behaviors the settings rows depend on:
 #
 #   commitPreference(key, value)
 #     One entry point for every row. A compositor-backed key must reach Hyprland
@@ -11,7 +11,7 @@
 #
 #   restoreDefaults()
 #     Panama keeps user state in more than one file. Resetting only the schema
-#     store would leave a customised Home accessory arrangement in place while
+#     store would leave a customized Home accessory arrangement in place while
 #     claiming to have restored Panama's defaults. That is worse than having no
 #     reset at all, because it is silent.
 
@@ -168,7 +168,7 @@ done
 
 home_after="$(qs_for_harness ipc call settings-system-test homeState)"
 jq -e '.count == 0 and .initialized == false' <<<"$home_after" >/dev/null \
-    || fail "reset left the Home accessory store customised: $home_after"
+    || fail "reset left the Home accessory store customized: $home_after"
 
 # Resetting a stored value does not itself apply compositor policy, so the last
 # isolated batch must contain the shipped default.