feat: multi-network join, persistence, and switching in web UI

- Sidebar: "+" button next to Networks opens an inline join form;
  in browser mode shows anchor URL field; saves to waste_saved_networks
- Onboarding: saved networks shown as quick-join chips above the join
  form; auto-rejoin all saved networks on reconnect (not just one)
- Browser mode: networks persisted as [{name, anchor}] array in
  localStorage (waste_saved_networks); legacy waste_last_network kept
  for backward compat
- Daemon mode: daemon already tracks joined networks; web UI just needs
  the "+" to send join_network for additional ones

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

web/src/App.css

@@ -33,6 +33,10 @@ input:focus { border-color: var(--accent); }
 .onboarding-section { width: 100%; border-top: 1px solid var(--border); padding-top: 12px; }
 .join-form { display: flex; flex-direction: column; gap: 8px; width: 100%; }
 .join-label { font-size: 11px; text-transform: uppercase; letter-spacing: 0.08em; color: var(--muted); }
+.saved-networks { display: flex; flex-direction: column; gap: 6px; width: 100%; }
+.saved-network-chips { display: flex; flex-wrap: wrap; gap: 6px; }
+.network-chip { background: var(--surface); border: 1px solid var(--border); color: var(--text); border-radius: 20px; padding: 4px 14px; font-size: 13px; }
+.network-chip:hover { border-color: var(--accent); color: var(--accent); background: var(--surface); }
 button.primary { background: var(--accent); width: 100%; padding: 8px; font-size: 14px; }
 .toggle-link { background: none; color: var(--muted); font-size: 12px; padding: 4px 0; text-align: left; }
 .toggle-link:hover { color: var(--text); }