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/nvim/lua/plugins/colorscheme.lua

@@ -1,12 +1,12 @@
--- Tokyo Night, following the desktop's colour scheme.
+-- Tokyo Night, following the desktop's color scheme.
 --
 -- Moon when Panama is dark, Day when it is light. Same theme family either way,
 -- so the editor keeps the identity the rest of the desktop has rather than
 -- becoming a different-looking application when the scheme flips.
 --
 -- The readability fixes below are deliberately dark-only. They were written
--- against Moon's palette -- a pale comment colour, a mid-grey gutter -- and
--- applying them to Day would put light grey text on a light background, which
+-- against Moon's palette -- a pale comment color, a mid-gray gutter -- and
+-- applying them to Day would put light gray text on a light background, which
 -- is exactly the legibility problem they exist to solve, inverted.
 
 local panama = require("config.panama")