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/env.lua

@@ -1,7 +1,7 @@
 -- ─────────────────────────────────────────────────────────────────────────────
 -- Environment
 --
--- hl.env() sets variables before the display server initialises, which is what
+-- hl.env() sets variables before the display server initializes, which is what
 -- makes it valid for AQ_* and cursor variables.
 --
 -- NOTE for uwsm sessions: if you log in via "Hyprland (uwsm-managed)", uwsm
@@ -58,7 +58,7 @@ hl.env("QT_QPA_PLATFORM", "wayland;xcb")
 
 -- "gtk3" makes Qt derive its palette from the live GTK3 theme via the qgtk3
 -- platform plugin, so Qt apps match adw-gtk3-dark exactly rather than
--- approximating it -- and it gives them GTK file/font/colour dialogs. It also
+-- approximating it -- and it gives them GTK file/font/color dialogs. It also
 -- covers Qt5 and Qt6 with one value.
 --
 -- Note this is a single value, NOT a fallback list: Qt splits on ":" and uses