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/connectivity-contract.sh

@@ -69,7 +69,7 @@ fi
 # ── Signal strength is a ratio, and the labels must reflect that ─────────────
 while IFS='|' read -r value expect; do
     got="$(run ipc call connectivity-test labelFor "$value")"
-    [[ "$got" == "$expect" ]] || fail "signal $value labelled '$got', expected '$expect'"
+    [[ "$got" == "$expect" ]] || fail "signal $value labeled '$got', expected '$expect'"
 done <<'CASES'
 1.0|Excellent
 0.85|Excellent