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

tests/quickshell/prism-gallery-contract.sh

@@ -26,7 +26,7 @@ rg -Fq 'FloatingWindow {' "$gallery" \
     || fail 'gallery must be a normal tiled application window'
 
 if rg -q '#[0-9a-fA-F]{3,8}' "$gallery"; then
-    fail 'gallery hardcodes a colour instead of using the Theme vocabulary'
+    fail 'gallery hardcodes a color instead of using the Theme vocabulary'
 fi
 if rg -q 'loops:[[:space:]]*(Animation\.Infinite|-1)|running:[[:space:]]*true' "$gallery"; then
     fail 'gallery contains an always-running animation'