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

docs/superpowers/specs/2026-08-17-home-accessories-customization-design.md

@@ -107,7 +107,7 @@ Devices and Desktop & Dock. Its page contains three cards.
 
 1. **Home Assistant** shows connection health, discovered-light count, Refresh,
    and Open Home Assistant.
-2. **Control Center favourites** shows selected accessories as a two-column
+2. **Control Center favorites** shows selected accessories as a two-column
    card shelf. Each item exposes a drag handle, editable alias, source friendly
    name, and Remove action. Dragging reorders the list; the first four are
    explicitly marked as the resting Control Center shelf.