Let a browser send the device fingerprint header it is offered

`fingerprint.ts` reads the device fingerprint from a request body or, for
authenticated requests with no body to carry it, from
`x-puter-device-fingerprint`. That header is documented there as "the
header the GUI may send the device fingerprint on for non-signup
requests", and the middleware has always honoured it.

A browser could never actually send it. The CORS allowlist in
`#installCors` does not include it, so the preflight comes back without
it in `Access-Control-Allow-Headers`, the browser abandons the request,
and `fetch` rejects before anything reaches the server. Nothing in tree
sends the header today, which is why this went unnoticed: the first
client to try it sees every call fail with a network error rather than a
readable status.

Adds the header to the allowlist, and a preflight test asserting the
browser is allowed to ask for it.

src/backend/server.test.ts

@@ -272,6 +272,29 @@ describe('PuterServer host header validation', () => {
         );
     });
 
+    it('lets a cross-origin preflight ask for the device fingerprint header', async () => {
+        // `fingerprint.ts` reads the device fingerprint off
+        // `x-puter-device-fingerprint` for authenticated requests with no body
+        // to carry it. That channel only exists if the preflight admits the
+        // header: a browser abandons the request otherwise, and the call never
+        // leaves it.
+        const res = await request(
+            '/healthcheck',
+            {
+                host: `api.puter.localhost:${port}`,
+                origin: 'http://puter.localhost',
+                'access-control-request-method': 'GET',
+                'access-control-request-headers':
+                    'authorization,x-puter-device-fingerprint',
+            },
+            'OPTIONS',
+        );
+        expect(res.status).toBe(200);
+        expect(
+            String(res.headers['access-control-allow-headers']).toLowerCase(),
+        ).toContain('x-puter-device-fingerprint');
+    });
+
     it('still short-circuits OPTIONS preflight off the dav subdomain', async () => {
         const res = await request(
             '/some-path',

src/backend/server.ts

@@ -685,6 +685,12 @@ export class PuterServer {
             'X-Expected-Entity-Length',
             'DAV',
             'stripe-signature',
+            // The GUI's fallback channel for the device fingerprint on
+            // authenticated requests that have no body to carry it (see
+            // core/http/middleware/fingerprint.ts). Without it here the
+            // preflight refuses the header and the request never leaves the
+            // browser.
+            'x-puter-device-fingerprint',
         ].join(', ');
 
         // What a browser DAV client is allowed to read back off a response.