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>

web/src/pages/Onboarding.tsx

@@ -14,11 +14,17 @@ interface Props {
 //   ?a=<url>              anchor URL hint
 function parseInviteParams(): { network: string; netHash: string; anchor: string; inviteString: string } {
   const p = new URLSearchParams(window.location.search)
-  const inviteString = p.get('invite') ?? ''
+  let inviteString = p.get('invite') ?? ''
   let network = p.get('n') ?? p.get('network') ?? ''
   let netHash = p.get('net') ?? ''
   let anchor = p.get('a') ?? p.get('anchor') ?? ''
 
+  // Hash-based hang link: https://host/#waste:eyJ... (opaque, not sent to server)
+  const hash = window.location.hash.slice(1) // strip leading #
+  if (!inviteString && hash.startsWith('waste:')) {
+    inviteString = hash
+  }
+
   if (inviteString.startsWith('waste:')) {
     try {
       const json = JSON.parse(atob(inviteString.slice(6).replace(/-/g, '+').replace(/_/g, '/')))