fix(editor): File→Quit returns to the management app it was entered from

The quit hook only history.back()ed on a SAME-ORIGIN referrer, so the
primary closed-deploy entry — app.pcbjam.com deep-linking into
editor.pcbjam.com (cross-origin) — fell through to the editor's own
project view instead of returning to the app. Quit is Back-button
semantics (3a2255c): any non-empty referrer with in-tab history now
goes back; deep links / fresh tabs (no referrer) keep the project-page
fallback. Verified live against the closed stack: :3047 → editor →
quit lands back on :3047; a direct editor URL still falls back to the
editor's project overview.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/web/quit-to-project.spec.ts

@@ -73,8 +73,8 @@ test.describe('web app — File → Quit leaves the editor', () => {
   test('quit on a deep-linked editor falls back to the project page', async ({ page }) => {
     test.setTimeout(300000);
 
-    // Direct entry: no same-origin referrer, nothing meaningful to go back to —
-    // quit must land on the project page via the fallback URL.
+    // Direct entry (typed URL): no referrer, nothing meaningful to go back
+    // to — quit must land on the project page via the fallback URL.
     await page.goto(`/${SCOPE}/projects/demo/demo.kicad_sch`);
     await waitForToolReady(page, /demo — Schematic Editor/i);