fix(smtp): proper SMTP dialogue, encoded mails, optional skip-verify

- Explicit SMTP flow (dial -> EHLO -> STARTTLS -> AUTH -> MAIL/RCPT/DATA)
  with errors naming the refusing step (also logged as smtp_step)
- EHLO with the sender's domain instead of "localhost"
- Envelope gets the bare address; "Name <addr>" stays in the From header
  (was rejected as invalid MAIL FROM)
- Date and Message-ID headers, RFC 2047 subject, quoted-printable body
- SMTP_SKIP_VERIFY / admin switch for self-signed certificates;
  SMTP checkboxes are switches now
- Tests against a fake SMTP server (STARTTLS, 451, display names)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Doku/architecture.md

@@ -64,6 +64,7 @@ Das Frontend liegt per `go:embed` im Binary ([cmd/server/main.go](../cmd/server/
 | `LOG_LEVEL` / `DEBUG` | `info` / `false` | `debug`, `info`, `warn`, `error`; `DEBUG=true` erzwingt `debug` |
 | `MINDA_DEMO` (auch `MINDSIFT_DEMO`, `DEMO_MODE`) | `false` | Demo-Konten anlegen |
 | `SMTP_HOST`, `SMTP_PORT`, `SMTP_USER`, `SMTP_PASS`, `SMTP_FROM`, `SMTP_TLS` | leer / `587` / `true` | Mailversand, auch im Admin-UI einstellbar |
+| `SMTP_SKIP_VERIFY` | `false` | Zertifikat des Mailservers nicht prüfen (nur interne Server mit selbstsigniertem Zertifikat) |
 
 Hinweis: `DATABASE_URL` mit `postgres…` führt dazu, dass der Standardpfad in `DATA_DIR` genutzt wird; Postgres ist nicht implementiert.