Harden server security, rework landing page and refresh branding

Security:
- rooms: rate-limit /invite-email (SMTP spam relay), validate share
  target user exists, guard timingSafeEqual against length mismatch
  in the presentation route (500 -> 403)
- analytics: verify callback token before parsing the 5mb body so
  unauthenticated callers cannot buffer large payloads
- caldav: rate-limit failed Basic-Auth attempts (token brute force),
  lowercase email lookup, case-insensitive principal check
- auth: fall back to the in-memory rate-limit store when Redis is
  unavailable; previously every rate-limited endpoint (incl. login)
  returned 500 when the Redis connection was down

UI/copy:
- Home: factual hero copy and feature cards (6 instead of 9), fix
  double-rendered feature icon, remove fake stats row and pill badge;
  keep the background gradient and card layout
- i18n: consistent informal tone, drop trailing exclamation marks
  from status toasts, remove emoji from transactional emails
- new favicon (logo.svg), restore theme-based default brand logo

Chore:
- gitignore SQLite WAL/SHM files

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

server/i18n/de.json

@@ -1,6 +1,6 @@
 {
   "email": {
-    "greeting": "Hey {name} 👋",
+    "greeting": "Hallo {name},",
     "viewInvitation": "Einladung anzeigen",
     "invitationFooter": "Öffne den Link oben, um die Einladung anzunehmen oder abzulehnen.",
     "linkHint": "Oder kopiere diesen Link in deinen Browser:",
@@ -13,7 +13,7 @@
     },
     "invite": {
       "subject": "{appName} - Du wurdest eingeladen",
-      "title": "Du wurdest eingeladen! 🎉",
+      "title": "Du wurdest eingeladen",
       "intro": "Du wurdest eingeladen, ein Konto auf {appName} zu erstellen.",
       "prompt": "Klicke auf den Button, um dich zu registrieren:",
       "button": "Konto erstellen",