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/hypr/prefs-fallback-contract.sh

@@ -4,7 +4,7 @@
 # monitors.lua. It is therefore the one piece of Panama that can cost the user a
 # working compositor rather than merely a working feature.
 #
-# This contract pins the only behaviour that matters: whatever is in the
+# This contract pins the only behavior that matters: whatever is in the
 # settings file -- including nothing, garbage, or values of the wrong type --
 # the config still parses, and every setting either takes the stored value or
 # falls back to the value shipped in the Lua.
@@ -106,7 +106,7 @@ result="$(probe '{"note": "a \"quoted\" value\nwith escapes", "nested": {"a": [1
 # defaults is only useful if Hyprland still accepts the config around it.
 #
 # Hyprland resolves its own config from $HOME/.config/hypr regardless of
-# XDG_CONFIG_HOME, while prefs.lua honours XDG_CONFIG_HOME. That asymmetry is
+# XDG_CONFIG_HOME, while prefs.lua honors XDG_CONFIG_HOME. That asymmetry is
 # what makes this loop useful rather than vacuous: the *real* looks.lua and
 # input.lua are parsed against a *fixture* settings file. The probe cases above
 # already establish that prefs.lua reads the fixture and not the live file.