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/hypr/input.lua

@@ -22,10 +22,10 @@ hl.config({
         repeat_rate  = prefs.get("keyRepeatRate", 33),
 
         -- 1 = FOLLOW. The window under the pointer takes focus. This comment
-        -- previously claimed 1 was "click to focus, GNOME's behaviour", which
+        -- previously claimed 1 was "click to focus, GNOME's behavior", which
         -- is the opposite of what Hyprland does -- `hyprctl descriptions` gives
         -- map: [{"separate":3},{"detached":2},{"follow":1},{"disabled":0}],
-        -- so click-to-focus is 0. Changing the shipped value is a behaviour
+        -- so click-to-focus is 0. Changing the shipped value is a behavior
         -- decision rather than a correction, so the value is left alone and
         -- only the description is fixed; Settings exposes all four.
         follow_mouse = prefs.getInt("followMouse", 1),