Add SOAP 1.1 API alongside REST + two-layer grouped nav

[Copilot]

Adds a SOAP 1.1 endpoint exposing the same email operations as the REST API, and reorganises the sidebar navigation into a two-layer grouped hierarchy.

SOAP API

New GET /api/soap (WSDL) and POST /api/soap (operations) endpoints. Auth is identical to REST (Basic Auth or ******

Operations: ListEmails · GetEmail · SendEmail · DeleteEmail

POST /api/soap
Authorization: ******
Content-Type: text/xml; charset=utf-8
SOAPAction: "urn:mailserver#ListEmails"

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <ListEmails xmlns="urn:mailserver">
      <accountId>1</accountId>
      <folder></folder>
      <page>1</page>
    </ListEmails>
  </soap:Body>
</soap:Envelope>

• SOAP envelope parsing via quick-xml (already a dependency); no new crates added
• Faults returned as standard soap:Fault with soap:Client / soap:Server codes
• WSDL 1.1 document served at GET /api/soap
• API docs page (/api) updated with SOAP operation table and request/response examples

Two-layer navigation

Flat 21-item list replaced with Dashboard (standalone) + 5 logical groups:

Group	Items
Mail	Domains, Accounts, Aliases, Forwarding, Relays
Processing	Queue, Tracking, Unsubscribe, Spambl, DMARC, Abuse, Fail2ban
Clients	Webmail, CalDAV, WebDAV
Integration	API, Webhooks, MCP
System	Configs, Settings

Group labels (.nav-group-label) are uppercase section headers. On the desktop sidebar child links are indented and each group is separated by a subtle rule via .nav-group in style.css / desktop.css.

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