fix(dev): Vite-API-Proxy, Auth, Stripe-Mails und Backend-Erweiterungen

- Client: API-Basis-URL (joinApiUrl, /v1-Falle), Vite strictPort + Proxy 127.0.0.1, Nicht-JSON-Fehler

- Server: /api-404 ohne Wildcard-Bug, SPA-Fallback, Auth-Middleware, Cron, Mailer, Crypto

- Routen: OAuth-State, Email/Stripe/Analytics; client/.env.example

Made-with: Cursor

server/emails/subscription-updated.txt

@@ -0,0 +1,9 @@
+Hello,
+
+Your MailFlow subscription was updated.
+
+Plan: {{plan}}
+Status: {{status}}
+{{periodEndLine}}
+
+— MailFlow