feat(mcp-server): add theme menu, dark mode and editable SVG to the preview

- Pass themes=1 and dark=auto to the draw.io iframe, so the Extras menu
  offers the Theme submenu and draw.io follows the system dark mode; the
  header and dialogs follow it too through prefers-color-scheme
- Download dialog: new Editable SVG (.drawio.svg) format through draw.io's
  xmlsvg export, default name diagram-YYYY-MM-DD, Enter saves and Escape
  closes it and the history dialog
- export_diagram accepts format "drawio.svg" and detects the .drawio.svg
  extension
- Use system fonts instead of Google Fonts, so the page works offline with
  a self-hosted draw.io (DRAWIO_BASE_URL)

packages/mcp-server/src/http-server.ts

@@ -104,12 +104,15 @@ interface SessionState {
     lastPolled?: number // Last browser poll; an open tab keeps the session alive
     svg?: string // Cached SVG from last browser save
     syncRequested?: number // Timestamp when sync requested, cleared when browser responds
-    exportFormat?: "png" | "svg" // Set by MCP tool to request browser export
+    exportFormat?: ExportFormat // Set by MCP tool to request browser export
     exportXml?: string // Single-page projection to load before a page-targeted export
     exportOptions?: ExportOptions // Extra draw.io export parameters (PNG only)
     exportData?: string // Base64/SVG data returned by browser after export
 }
 
+/** draw.io export formats; xmlsvg is an SVG with the diagram embedded */
+export type ExportFormat = "png" | "svg" | "xmlsvg"
+
 /**
  * draw.io's PNG export takes these directly: width caps the image size
  * (never upscales), pageId renders a page other than the one on screen.
@@ -179,7 +182,7 @@ export function setState(
  */
 export function requestExport(
     sessionId: string,
-    format: "png" | "svg",
+    format: ExportFormat,
     projectionXml?: string,
     options?: ExportOptions,
 ): boolean {