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/looks.lua

@@ -1,7 +1,7 @@
 -- ─────────────────────────────────────────────────────────────────────────────
 -- Look and feel -- Tokyo Night Moon
 --
--- Colours here must stay in sync with quickshell/config/Theme.qml.
+-- Colors here must stay in sync with quickshell/config/Theme.qml.
 --   accent  user-selectable, see `accents` below -- blue (#82aaff) ships
 --   bg      #222436   base
 --
@@ -53,7 +53,7 @@ hl.config({
             -- from Theme.accent/Theme.accentSecondary on every scheme change
             -- too, since each accent carries a separate pair per scheme.
             active_border   = { colors = { "rgba(" .. accentStart .. "ee)", "rgba(" .. accentEnd .. "ee)" }, angle = 115 },
-            -- The neutral inactive role follows the colour scheme because a
+            -- The neutral inactive role follows the color scheme because a
             -- dark neutral disappears against a light desktop.
             -- services/ColorScheme.qml applies the same values live; this is
             -- the value a fresh session starts from.
@@ -193,12 +193,12 @@ hl.config({
         -- Let a crashed lock screen be recovered rather than stranding you.
         allow_session_lock_restore = true,
 
-        -- Don't let apps steal focus by shouting; matches GNOME's behaviour.
+        -- Don't let apps steal focus by shouting; matches GNOME's behavior.
         focus_on_activate = false,
     },
 
     render = {
-        -- Colour management is stable and on by default in 0.56.
+        -- Color management is stable and on by default in 0.56.
         cm_enabled = true,
 
         -- 1 = automatically flip the monitor into HDR for fullscreen content