Add unsafe raw-nsec login fallback

Collapsed-by-default 'paste an nsec (unsafe)' option on the login screen,
signs the NIP-98 login event client-side with nostr-tools and never
persists the key (used once in memory, discarded) — for testing/kiosk
use without a NIP-07 extension. Clearly labeled as unsafe in the UI.

frontend/src/lib/nsec.ts

@@ -0,0 +1,45 @@
+// Raw-nsec signing — an *unsafe* login fallback for when there's no NIP-07
+// extension or NIP-46 signer available (testing, kiosk devices, etc). The
+// private key only ever lives in page memory for the duration of one sign
+// call; it's never persisted (no localStorage/sessionStorage), matching how
+// the NIP-07 path also never retains key material — only the resulting
+// session cookie survives.
+import { nip19 } from 'nostr-tools';
+import { finalizeEvent, getPublicKey } from 'nostr-tools/pure';
+import type { SignedEvent, UnsignedEvent } from './nip07';
+
+export function isValidNsec(nsec: string): boolean {
+  try {
+    return nip19.decode(nsec.trim()).type === 'nsec';
+  } catch {
+    return false;
+  }
+}
+
+function decodeNsec(nsec: string): Uint8Array {
+  const decoded = nip19.decode(nsec.trim());
+  if (decoded.type !== 'nsec') throw new Error('Not an nsec key');
+  return decoded.data;
+}
+
+export function nsecPubkey(nsec: string): string {
+  return getPublicKey(decodeNsec(nsec));
+}
+
+function signWithNsec(nsec: string, event: UnsignedEvent): SignedEvent {
+  return finalizeEvent(event, decodeNsec(nsec));
+}
+
+/** Sign a NIP-98 (kind 27235) event with a raw nsec and return the Authorization header value. */
+export function buildNip98HeaderWithNsec(nsec: string, url: string, method: string): string {
+  const event = signWithNsec(nsec, {
+    kind: 27235,
+    created_at: Math.floor(Date.now() / 1000),
+    content: '',
+    tags: [
+      ['u', url],
+      ['method', method],
+    ],
+  });
+  return `Nostr ${btoa(JSON.stringify(event))}`;
+}