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/health-ui-contract.sh

@@ -65,7 +65,7 @@ rg -Fq 'SystemSettings.openGnomePanel("system", "users")' "$settings_dir/HealthP
 rg -Fq 'SystemSettings.openGnomePanel("sharing")' "$settings_dir/HealthPage.qml" \
     || fail 'Fedora ownership boundary lost the Sharing handoff'
 rg -Fq 'SystemSettings.openGnomePanel("color")' "$settings_dir/HealthPage.qml" \
-    || fail 'Fedora ownership boundary lost the Colour profiles handoff'
+    || fail 'Fedora ownership boundary lost the Color profiles handoff'
 rg -Fq 'SystemSettings.openGnomePanel("wellbeing")' "$settings_dir/HealthPage.qml" \
     || fail 'Fedora ownership boundary lost the Digital wellbeing handoff'
 # Exact authored handoffs are asserted above. Also prove every panel named by
@@ -307,7 +307,7 @@ jq -e '
     and .fedoraHandoffs == [
         {id:"users", label:"Users", action:"Open users"},
         {id:"sharing", label:"Sharing", action:"Open sharing"},
-        {id:"color", label:"Colour profiles", action:"Open colour"},
+        {id:"color", label:"Color profiles", action:"Open color"},
         {id:"wellbeing", label:"Digital wellbeing", action:"Open wellbeing"}
     ]
     and .summaryHeight == 126