fix(mcp-server): review fixes for the preview URL and the theme menu

The stalled-tab note names the page start_session opened (the shell
with DRAWIO_PREVIEW_UI=shell) instead of the classic page. With the
shell not built, start_session opens the classic page and the server
says so at startup, as it does for a missing dist/drawio. BROWSER=none
skips the system browser, as Vite and CRA dev servers do: the e2e tests
set it. Without a fixed DRAWIO_UI, the shell's draw.io URL carries
themes=1 so Extras > Theme is offered, as on the classic page.

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

@@ -122,7 +122,9 @@ export function previewUrl(
     sessionId: string,
     ui: PreviewUi = PREVIEW_UI,
 ): string {
-    const path = ui === "shell" ? "/shell/" : ""
+    // The classic page stands in while the shell is not built (a run from
+    // the sources), as embed.diagrams.net does for a missing dist/drawio
+    const path = ui === "shell" && shellDir !== null ? "/shell/" : ""
     return `http://localhost:${port}${path}?mcp=${sessionId}`
 }
 
@@ -430,6 +432,11 @@ export function startHttpServer(port = 6002): Promise<number> {
                     "No bundled draw.io (dist/drawio missing); the preview loads it from embed.diagrams.net",
                 )
             }
+            if (PREVIEW_UI === "shell" && shellDir === null) {
+                log.warn(
+                    "The canvas shell is not built (dist/shell missing); start_session opens the classic page",
+                )
+            }
             resolve(port)
         })
     })

packages/mcp-server/src/index.ts

@@ -382,9 +382,10 @@ registerWriteTool(
                 lastSeenXml: "",
             }
 
-            // Open browser
+            // Open the browser. BROWSER=none (the convention of CRA and Vite
+            // dev servers) skips it: the e2e tests and headless hosts set it
             const browserUrl = previewUrl(port, sessionId)
-            await open(browserUrl)
+            if (process.env.BROWSER !== "none") await open(browserUrl)
 
             // A saved diagram comes back from its file. lastSeenXml stays
             // empty, so edit_diagram asks for get_diagram first.
@@ -1287,7 +1288,7 @@ function previewStalled(sessionId: string): boolean {
  * already succeeded must not run again, so it points to screenshot_diagram.
  */
 function previewStalledNote(sessionId: string, next = "then retry") {
-    return `The preview tab is not responding (browsers pause background tabs). Ask the user to bring the preview tab to the front (http://localhost:${getServerPort()}?mcp=${sessionId}), ${next}.`
+    return `The preview tab is not responding (browsers pause background tabs). Ask the user to bring the preview tab to the front (${previewUrl(getServerPort(), sessionId)}), ${next}.`
 }
 
 function previewStalledError(sessionId: string) {