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/migrations-contract.sh

@@ -4,7 +4,7 @@
 #
 # The schema says what a setting IS; it cannot say what a setting USED to be.
 # Rename a key, change its units, or split one setting into two, and the stored
-# value stops being recognised -- and unrecognised keys are deliberately carried
+# value stops being recognized -- and unrecognized keys are deliberately carried
 # through untouched, so the user's choice silently stops taking effect with
 # nothing to explain it.
 #
@@ -67,6 +67,6 @@ check '.failure.v == 3 and .failure.count == 2 and .failure.kept == true' \
 
 # The promise that makes rollback safe.
 check '.preserved.kept == true' \
-    'a migration must not discard keys it does not recognise' '.preserved'
+    'a migration must not discard keys it does not recognize' '.preserved'
 
 printf 'migrations contract: PASS\n'