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/dock/DockBody.qml

@@ -172,14 +172,14 @@ Rectangle {
             }
         }
 
-        // Centred on the hovered item, clamped inside the dock. Reads the item's
+        // Centered on the hovered item, clamped inside the dock. Reads the item's
         // x directly (rather than mapToItem) so the binding re-evaluates when
         // the row reflows.
         x: {
             if (!root.hoveredItem)
                 return 0;
-            const centre = row.x + root.hoveredItem.x + root.hoveredItem.width / 2;
-            return Math.max(4, Math.min(root.width - width - 4, centre - width / 2));
+            const center = row.x + root.hoveredItem.x + root.hoveredItem.width / 2;
+            return Math.max(4, Math.min(root.width - width - 4, center - width / 2));
         }
         y: -height - 8