feat(web): admin area with instance default theme, custom logo/favicon; .theme.json export

- Admin settings tab (admin-only) now holds user management plus a server-wide
  default theme editor (live preview + discard) and logo/favicon branding.
- New singleton InstanceSettings model + /api/instance router (public GET for
  the login screen; admin-gated theme PUT and logo/favicon upload/delete,
  reusing the avatar PIL/validation pattern).
- Instance default theme is the base users inherit and the target a per-colour
  "Reset" returns to (baseColor: instance default -> built-in).
- Custom favicon overrides the primary-colour tinting; custom logo replaces the
  top-left glyph+text, hard-capped so it only scales down and never breaks layout.
  Branding + defaults load before login (public endpoint).
- Theme export now uses the recognised .theme.json extension (old .theme still
  imports); import stays partial/unknown-key aware.

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

frontend/sw.js

@@ -7,7 +7,7 @@
 // the entry HTML / version files). New releases take effect on the next
 // reload, no manual SW unregister required.
 
-const CACHE_VERSION  = 'calendarr-v32';
+const CACHE_VERSION  = 'calendarr-v33';
 const OFFLINE_SHELL  = ['/', '/index.html'];
 
 self.addEventListener('install', event => {