docs: improve .env.example with better placeholders and comments

- Use controlplane.tailscale.com as default TS_LOGIN_SERVER
- Use consistent .your-tailnet.ts.net and .yourdomain.com placeholders
- Add clear section comments

.env.example

@@ -0,0 +1,22 @@
+# ==========================================
+# Tailscale / Headscale Configuration
+# ==========================================
+
+# Your Tailscale or Headscale auth key
+# Generate this in the admin console (make it reusable if needed)
+TS_AUTHKEY=***
+
+# Tailscale control server (default) or your Headscale URL
+TS_LOGIN_SERVER=https://controlplane.tailscale.com
+
+# ==========================================
+# Application-Specific Configuration
+# ==========================================
+
+# Public URL users will access the service at
+# For Headscale: usually http://vikunja.yourdomain.com
+# For Tailscale with certs: usually https://vikunja.your-tailnet.ts.net
+VIKUNJA_SERVICE_PUBLICURL=http://vikunja.your-tailnet.ts.net
+
+# Frontend URL (usually the same as PUBLICURL)
+VIKUNJA_SERVICE_FRONTENDURL=http://vikunja.your-tailnet.ts.net