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/monitors.lua

@@ -25,7 +25,7 @@ local prefs = require("prefs")
 --       x = 0, y = 0, primary = true,
 --   } }
 --
--- Only mode, scale, and transform are read. Colour management and bit depth
+-- Only mode, scale, and transform are read. Color management and bit depth
 -- stay here, because those are the settings with a documented reason attached
 -- (see the header) rather than preferences, and a settings page has no way to
 -- explain the screencopy tradeoff at the moment you would be changing it.
@@ -147,7 +147,7 @@ hl.monitor({
 })
 
 -- Other connected outputs use the same validated per-output store. They keep
--- automatic placement and the compositor's normal colour policy; DP-2 alone
+-- automatic placement and the compositor's normal color policy; DP-2 alone
 -- carries the panel-specific 10-bit policy documented above.
 for output, _ in pairs(displays) do
     if output ~= "DP-2" then