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/referrer-policy.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect } from "vitest";
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+/**
+ * Regression guard for the QR-login / cookie-save outage (and in fact every
+ * mutating WebUI action). On 2026-05-27 the WebUI-auth feature added the
+ * same-origin CSRF gate `app.use("/api", csrfOriginCheck)` in server.ts, and
+ * the same day a `<meta name="referrer" content="no-referrer">` was added to
+ * web/index.html so cross-origin CDN cover thumbnails would load.
+ *
+ * Those two changes conflict: per the WHATWG Fetch "Append a request Origin
+ * header" algorithm, the `no-referrer` policy sets the Origin header to the
+ * literal string "null" on same-origin non-GET requests. csrfOriginCheck then
+ * fails to parse a host (`new URL("null")` throws) and returns 403 "bad
+ * origin", so POST /api/auth/qrcode (and every other POST/PUT/DELETE under
+ * /api/* except /api/session/*) never reaches its handler.
+ *
+ * `same-origin` is the correct policy: it keeps the real Origin on same-origin
+ * requests (CSRF passes) while still sending no Referer cross-origin (CDN
+ * thumbnails keep loading). Never switch this back to `no-referrer`.
+ */
+describe("frontend referrer policy (CSRF / Origin-header regression)", () => {
+  const indexHtmlPath = path.resolve(
+    path.dirname(fileURLToPath(import.meta.url)),
+    "../../web/index.html"
+  );
+  const html = fs.readFileSync(indexHtmlPath, "utf-8");
+
+  const referrerMeta = html.match(
+    /<meta\s+name=["']referrer["']\s+content=["']([^"']+)["']\s*\/?>/i
+  );
+
+  it("declares a referrer policy meta tag", () => {
+    expect(referrerMeta).not.toBeNull();
+  });
+
+  it("uses same-origin (NOT no-referrer, which sends Origin: null and 403s every POST)", () => {
+    expect(referrerMeta?.[1]).toBe("same-origin");
+  });
+
+  it("does not contain no-referrer anywhere in the document head", () => {
+    expect(html).not.toMatch(/content=["']no-referrer["']/i);
+  });
+});