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/quickshell/modules/settings/SliderRow.qml

@@ -4,7 +4,7 @@
 //
 // Range, step, label, explanation, and unit all come from PreferenceSchema.
 //
-// Three behaviours worth knowing:
+// Three behaviors worth knowing:
 //
 //   * The row is responsive. The Settings window is a normal tiled window, so
 //     its width is whatever the layout gives it -- anywhere from a half-screen
@@ -63,7 +63,7 @@ Item {
 
     // A fixed trailing width rather than a share of the row. A proportional
     // control looks reasonable at 900px and absurd at 4500px, where the slider
-    // would be a metre long next to a two-word label -- and this window is
+    // would be a meter long next to a two-word label -- and this window is
     // tiled, so it really can be that wide.
     readonly property int controlSpan: 300