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/local/share/icons/panama-settings.svg

@@ -2,14 +2,14 @@
 <!--
   Panama Settings.
 
-  A gear, because a settings icon has to be recognisable at 48px in a dock
+  A gear, because a settings icon has to be recognizable at 48px in a dock
   before it is clever — but rendered in the Prism identity rather than the flat
   monochrome of the symbolic icon this replaces. Blue leads into orchid across
   the gradient, the same pair that marks the focused window, the active
   workspace and the hairline on every glass surface.
 
   Drawn on the dark tile (#222436) so it reads as an application icon among
-  full-colour ones, rather than as a UI glyph that wandered out of a toolbar.
+  full-color ones, rather than as a UI glyph that wandered out of a toolbar.
 -->
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128" width="128" height="128">
   <defs>