Add signed invites, hang links, multi-share, and EXTENSIONS.md

- Signed invites: waste: URI gains inviter+sig fields (Ed25519); hello
  carries the invite so receiving peers can verify against known keys
- RequireInvite per-network flag: rejects peers without valid signed invite
- Hash-based hang links: #waste:base64 fragment pre-fills join form without
  server-side leakage of network name
- Multi-share: shares.json (daemon) + waste_shares localStorage (browser);
  IPC add_share/remove_share/list_shares commands
- EXTENSIONS.md: addendum documenting all waste-go protocol deviations from
  YAW/2; all extensions are additive and backward compatible

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

internal/crypto/crypto.go

@@ -131,6 +131,9 @@ func (id *Identity) PeerInfo() proto.PeerInfo {
 	}
 }
 
+// PeerIDHex satisfies the invite.Signer interface.
+func (id *Identity) PeerIDHex() string { return string(id.PeerID()) }
+
 // Sign signs data with our Ed25519 private key. Returns hex-encoded signature.
 func (id *Identity) Sign(data []byte) string {
 	sig := ed25519.Sign(id.privateKey, data)