fix: replace X25519 with ECDH P-256 for Electron compatibility

X25519 Web Crypto support is not available in Electron's Chromium build
despite being in Chrome 113+. P-256 ECDH is universally supported across
all browsers and Electron versions with equivalent security.

- Replace { name: 'X25519' } with { name: 'ECDH', namedCurve: 'P-256' }
  throughout the keypair generation, sealing, and opening functions
- Wire format updated: ephemeral pub key grows from 32 to 65 bytes
  (uncompressed P-256 point: 0x04 || x || y)
- Existing accounts with X25519 keys must re-register

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/RegisterPage.tsx

@@ -60,7 +60,7 @@ export default function RegisterPage() {
       socket.connect(accessToken);
       navigate('/messages');
     } catch (err: any) {
-      setError(err.response?.data?.error || 'Registration failed');
+      setError(err.response?.data?.error || err.message || 'Registration failed');
     } finally {
       setLoading(false);
     }