Add browser pane element picker (#670)

* feat: add browser pane element picker

Port the browser pane from PR #198 and route picked elements through workspace attachments so the context follows every agent composer in a workspace.

Co-authored-by: Jason Kneen <jason.kneen@bouncingfish.com>

* docs: document electron platform overlays

* fix: harden browser pane navigation

* fix: polish browser tab interactions

* fix: route browser pane focus shortcuts

---------

Co-authored-by: Jason Kneen <jason.kneen@bouncingfish.com>

packages/desktop/src/features/browser-webviews.ts

@@ -0,0 +1,17 @@
+import type { WebContents } from "electron";
+
+const browserIdsByWebContentsId = new Map<number, string>();
+
+export function registerPaseoBrowserWebContents(contents: WebContents, browserId: string): void {
+  browserIdsByWebContentsId.set(contents.id, browserId);
+  contents.once("destroyed", () => {
+    browserIdsByWebContentsId.delete(contents.id);
+  });
+}
+
+export function getPaseoBrowserIdForWebContents(contents: WebContents | null): string | null {
+  if (!contents || contents.isDestroyed()) {
+    return null;
+  }
+  return browserIdsByWebContentsId.get(contents.id) ?? null;
+}