feat(web): recolour calendars shared with me (per-device local override)

Shared calendars belong to someone else, so a server colour PUT would 403.
Instead store the recipient's chosen colour locally (localStorage 'sharedCalColors',
keyed by calendar id) and re-apply it to events on every fetch. The colour dot
on a shared calendar now opens the picker instead of showing an error.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v72';
+export const APP_VERSION = 'v73';