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/tmux/themes/tokyonight-day.conf

@@ -2,7 +2,7 @@
 #
 # Same structure, same roles, Day's palette. Mapped role for role rather than by
 # hue: #1b1d2b was text drawn ON an accent block, so it becomes the LIGHT ground
-# rather than a dark colour, or the status bar would be dark text on a dark
+# rather than a dark color, or the status bar would be dark text on a dark
 # accent in light mode.
 #
 # The accents are Day's DARKER variants, not its standard ones. Measured against

config/dot/tmux/themes/tokyonight-moon.conf

@@ -3,7 +3,7 @@
 # Extracted from tmux.conf so the two schemes can be swapped. tmux.conf sources
 # current-theme.conf, which panama-theme-apps generates from one of these.
 #
-# Every colour here was already in tmux.conf; this file is that block verbatim.
+# Every color here was already in tmux.conf; this file is that block verbatim.
 
 set -g mode-style "fg=#82aaff,bg=#3b4261"