fix(mcp-server): review fixes for the shell page

- a download button in the header opens the web app's export dialog
  (.drawio, .png, .svg, .drawio.svg), which the classic page had and the
  shell lacked when it became the default
- the shell asks draw.io for the custom library menu (libraries=1), as
  the classic page did; the web app keeps libraries=0
- the newest card no longer shows "Rendering preview" for good: the sync
  takes the thumbnail of a diagram the server recovered from its file
  (saved without pictures) while the canvas kept it, and of a write whose
  picture was skipped because an edit came first, once the canvas shows
  the write again
- e2e: the get_selection test covers a shape in a container the user
  entered; a download test saves a .drawio file

lib/drawio/drawio-config.ts

@@ -21,6 +21,9 @@ export interface DrawioSource {
     lang?: string
     /** No calls to external services (the desktop app) */
     offline?: boolean
+    /** draw.io's custom libraries (File > New Library, Open Library from,
+     * and the buttons under the shapes); the web app leaves them out */
+    libraries?: boolean
 }
 
 /**
@@ -53,7 +56,7 @@ export function getDrawioSrc(source: DrawioSource, dark: boolean): string {
     else params.themes = "1"
     Object.assign(params, {
         spin: "0",
-        libraries: "0",
+        libraries: source.libraries ? "1" : "0",
         noSaveBtn: "1",
         noExitBtn: "1",
         saveAndExit: "0",