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>

FUTURE.md

@@ -121,6 +121,9 @@ Web frontend (React, already built) + Tauri shell for native packaging. The IPC
 | ✅ shipped | Session persistence + logout (browser mode) |
 | ✅ shipped | Persistent multi-share config (shares.json + localStorage) |
 | ✅ shipped | Subfolder support + directory browser UI in file browser |
+| ✅ shipped | Signed invites + invite-only networks (`RequireInvite`) |
+| ✅ shipped | Hash-based "come hang" links (`#waste:...`) |
+| ✅ shipped | Protocol extensions documented in EXTENSIONS.md |
 | next | TURN relay for daemon mode |
 | next | TUI room creation + daemon-side room persistence |
 | next | File transfer resume after disconnection |