Add support for custom HTTP headers in remote MCP servers

[christianwaldmann]

Closes #161 / #205

• Add custom headers configuration to UI (create/edit MCP server forms and detail page with show/hide toggle)
• Add headers column to mcp_servers table
• Include custom headers in SSE and Streamable HTTP transport requests (merged with auth headers)
• Handle headers in import/export functionality
• Update documentation (only English because I cannot speak Chinese)

Screenshots

Add server form:

Server details page:

Test plan

1. Create SSE/Streamable HTTP MCP server with custom headers
2. Verify headers display correctly on detail page
3. Verify headers are stored in database
4. Verify headers are sent in requests (tested with nc -nklp 3000)
5. Verify headers are preserved during import/export