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/services/SettingsSearch.qml

@@ -59,7 +59,7 @@ Singleton {
     // on a settings app that plainly has one.
     readonly property var extraEntries: [
         { label: "Timezone", detail: "Set the system timezone", page: "datetime" },
-        { label: "Network time", detail: "Synchronise the clock with a time server", page: "datetime" },
+        { label: "Network time", detail: "Synchronize the clock with a time server", page: "datetime" },
         { label: "Wi-Fi", detail: "Managed by GNOME Settings", page: "connectivity" },
         { label: "Bluetooth", detail: "Managed by GNOME Settings", page: "connectivity" },
         { label: "Printers", detail: "Managed by GNOME Settings", page: "connectivity" },
@@ -68,7 +68,7 @@ Singleton {
         { label: "Keyboard shortcuts", detail: "Every shortcut the compositor has bound", page: "shortcuts" },
         { label: "System Health", detail: "Check Panama services, integrations, tools, and recovery actions", page: "services" },
         { label: "Copy health report", detail: "Copy a redacted Panama doctor report", page: "services" },
-        { label: "Lock screen background", detail: "Choose a blurred desktop, wallpaper, or solid colour", page: "appearance" },
+        { label: "Lock screen background", detail: "Choose a blurred desktop, wallpaper, or solid color", page: "appearance" },
         { label: "Password field", detail: "Choose whether the empty lock-screen field stays visible", page: "appearance" },
         { label: "Per-display wallpaper", detail: "Assign a different image to each connected display", page: "appearance" },
         { label: "Arrange displays", detail: "Drag connected displays into their physical positions", page: "displays" },