fix(web): referrer-policy same-origin so same-origin POSTs keep a real Origin

no-referrer downgraded the Origin header to the literal "null" on same-origin non-GET requests (per the WHATWG Fetch "Append a request Origin header" algorithm), which the /api/* csrfOriginCheck then rejected with 403 "bad origin" — silently breaking QR login, cookie save, and every other WebUI POST/PUT/DELETE/PATCH (playback, bot management, user admin). /api/session/* was unaffected because it mounts before the CSRF gate, which is why WebUI login still worked.

same-origin keeps the real Origin on same-origin requests (CSRF passes) while still sending no Referer cross-origin, so B站/NetEase/QQ CDN cover thumbnails keep loading. Adds referrer-policy.test.ts pinning the policy and a csrf.test.ts case for the Origin: "null" rejection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/middleware/csrf.test.ts

@@ -55,4 +55,19 @@ describe("csrfOriginCheck middleware", () => {
       .set("Referer", "https://evil.com/some/path");
     expect(res.status).toBe(403);
   });
+
+  // Documents the server side of the QR-login outage: a `no-referrer` document
+  // policy makes the browser send the literal `Origin: null` on same-origin
+  // POSTs, which this guard cannot parse a host from and therefore rejects.
+  // The fix lives in the frontend (referrer policy -> same-origin); this test
+  // pins the gate behavior so the interaction stays understood. See
+  // src/web/referrer-policy.test.ts.
+  it('rejects POST with the literal Origin: "null" (no-referrer downgrade)', async () => {
+    const res = await request(app)
+      .post("/")
+      .set("Host", "example.com")
+      .set("Origin", "null");
+    expect(res.status).toBe(403);
+    expect(res.body).toEqual({ error: "bad origin" });
+  });
 });