feat(editor): preserve in-progress workspace across a login redirect

The Sign-in links navigate with a full page load (they mount in a
separate React root without Router context), which wipes the in-memory
Zustand workspace. New utils/workspaceDraft stashes the whole workspace
(reusing the lossless .vlx serialisation) to sessionStorage before that
navigation and restores it once when the editor remounts after login —
so a user who was building a circuit and signs in lands back on their
work instead of the empty starter board.

Strictly scoped to the login round-trip by a one-shot restore flag (not
a general autosave), and skipped when a named project is already loaded
so it never clobbers one. EditorPage calls restoreStashedWorkspace() on
mount; the pro overlay's auth links call stashWorkspaceForAuth() before
navigating.

frontend/src/pages/EditorPage.tsx

@@ -6,6 +6,7 @@ import React, { useRef, useState, useCallback, useEffect, lazy, Suspense } from
 import { useTranslation } from 'react-i18next';
 import { startSimulation } from '../simulation/spice/start';
 import { useSEO } from '../utils/useSEO';
+import { restoreStashedWorkspace } from '../utils/workspaceDraft';
 import { CodeEditor } from '../components/editor/CodeEditor';
 import { EditorToolbar } from '../components/editor/EditorToolbar';
 import { FileExplorer } from '../components/editor/FileExplorer';
@@ -100,6 +101,13 @@ export const EditorPage: React.FC = () => {
     return startSimulation();
   }, []);
 
+  // Restore an in-progress workspace stashed before a login redirect, so a
+  // user who was building something and signed in lands back on their circuit
+  // (not the empty starter board). One-shot; see utils/workspaceDraft.
+  useEffect(() => {
+    restoreStashedWorkspace();
+  }, []);
+
   // ── GitHub star prompt (show twice at most: 2nd visit OR after 3 min) ──────
   // Three localStorage flags drive this:
   //   velxio_star_prompted     → dismissed the first ask

frontend/src/utils/workspaceDraft.ts

@@ -0,0 +1,76 @@
+/**
+ * Workspace draft — keep an anonymous user's in-progress circuit + code across
+ * a full-page navigation to /login and back.
+ *
+ * The Sign-in links mount in a separate React root without Router context, so
+ * they navigate with `window.location.assign` (full page load), which wipes
+ * the in-memory Zustand stores. Before that navigation we stash the whole
+ * workspace (reusing the lossless `.vlx` serialisation) into sessionStorage;
+ * when the editor remounts after login it restores the stash. sessionStorage
+ * is same-tab and survives the reload, and is discarded when the tab closes.
+ *
+ * Scoped strictly to the login round-trip via a one-shot restore flag — it is
+ * NOT a general autosave, so a normal reload never resurrects an old draft.
+ */
+
+import type { BoardInstance } from '../types/board';
+import { useProjectStore } from '../store/useProjectStore';
+import { useSimulatorStore } from '../store/useSimulatorStore';
+import { buildVlxPayload } from './vlxFile';
+
+const DRAFT_KEY = 'velxio_ws_draft';
+const RESTORE_FLAG = 'velxio_ws_restore';
+
+/** True when the canvas holds something worth preserving (a real build, not
+ *  just the empty starter board). */
+export function workspaceHasWork(): boolean {
+  const sim = useSimulatorStore.getState();
+  return sim.components.length > 0 || sim.wires.length > 0;
+}
+
+/**
+ * Snapshot the current workspace and mark it for restore on the next editor
+ * mount. Call right before a full-page navigation to /login. No-op when there
+ * is nothing worth keeping or storage is unavailable.
+ */
+export function stashWorkspaceForAuth(): void {
+  try {
+    if (!workspaceHasWork()) return;
+    sessionStorage.setItem(DRAFT_KEY, JSON.stringify(buildVlxPayload()));
+    sessionStorage.setItem(RESTORE_FLAG, '1');
+  } catch {
+    // storage full / disabled — degrade to losing the draft, never throw.
+  }
+}
+
+/**
+ * If a stash is pending (set before the login redirect), load it into the
+ * stores and clear it. Runs once on editor mount. Skips when a named project
+ * is already loaded so it never clobbers one. Returns whether it restored.
+ */
+export function restoreStashedWorkspace(): boolean {
+  let raw: string | null = null;
+  try {
+    if (sessionStorage.getItem(RESTORE_FLAG) !== '1') return false;
+    sessionStorage.removeItem(RESTORE_FLAG);
+    raw = sessionStorage.getItem(DRAFT_KEY);
+    sessionStorage.removeItem(DRAFT_KEY);
+  } catch {
+    return false;
+  }
+  if (!raw) return false;
+  if (useProjectStore.getState().currentProject) return false;
+  try {
+    const payload = JSON.parse(raw);
+    useSimulatorStore.getState().loadProjectState({
+      boards: payload.boards as unknown as BoardInstance[],
+      fileGroups: payload.fileGroups,
+      components: payload.components,
+      wires: payload.wires,
+      activeBoardId: payload.activeBoardId,
+    });
+    return true;
+  } catch {
+    return false;
+  }
+}