feat: v0.7.2 — portmapper (UPnP+PCP+NAT-PMP), session-relay opt-in, URL Phase 1

Network/reachability improvements + a relay-privacy fix. Wire-compatible
with v0.7.0/v0.7.1; no protocol changes.

- Replace hand-rolled UPnP (igd-next) with the portmapper crate. All three
  protocols (UPnP-IGD / NAT-PMP / PCP) run in parallel, auto-renew
  internally. PCP adds IPv6 firewall pinholes and works on iOS without
  the multicast entitlement. Pulled in transitively via iroh already, no
  net dep growth.
- Android: UPnP/PCP/NAT-PMP attempted on WiFi/Ethernet with a
  WifiManager.MulticastLock acquired for the lifetime of the mapping.
  Cellular skipped early (no UPnP/PCP gateway, avoid 3s discovery waste).
- TCP port-mapping gate removed for mobile — phones with permissive NAT
  can now serve HTTP for direct browser fetches.
- Anchor reachability watcher (bidirectional): clears is_anchor after
  >5min of no port mapping; restores it when the mapping comes back.
  Network roams self-heal without restart. Mobile never auto-anchors.
- Session relay opt-in restored. relay.session_relay_enabled setting
  defaults OFF (anchors included — servers shouldn't silently burn
  bandwidth either). Gates both serving (can_accept_relay_pipe) and
  using (auto-fallback in node.rs). UI toggle in Settings. Relay-style
  signaling (RelayIntroduce / worm_lookup / N1-N3 shares) unaffected.
- URL Phase 1: share links now contain only the post ID
  (itsgoin.net/p/<post>). Anchor handler already supported post-ID-only
  URLs (author was optional); just dropped the author hex from the
  generator. Older URLs with author hex continue to work.
- Quick app close button in header (with confirm) — useful for stopping
  network activity between sessions on mobile.
- JNI null-pointer guards on ndk_context handles in android_wifi.rs.

MEMORY rule sharpened to distinguish session relay (byte pipe, opt-in)
from relay-style signaling/discovery (always on).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/style.css

@@ -18,6 +18,9 @@ header h1 { font-size: clamp(1.4rem, 2.5vw, 2rem); color: #7fdbca; margin: 0; }
 #net-dot.net-green { background: #22c55e; }
 #net-labels { font-size: 0.65rem; color: #888; display: flex; gap: 0.3rem; }
 .net-label { background: #2a2a40; padding: 0.1rem 0.35rem; border-radius: 3px; color: #aab; }
+#close-app-btn { margin-left: 0.6rem; padding: 0.3rem 0.5rem; background: transparent; border: 1px solid #444; border-radius: 4px; color: #999; font-size: 1rem; line-height: 1; cursor: pointer; }
+#close-app-btn:hover { color: #e74c3c; border-color: #e74c3c; }
+#close-app-btn:active { background: #2a1a1a; }
 
 /* Setup overlay */
 .overlay { position: fixed; top: 0; left: 0; right: 0; bottom: 0; background: rgba(10, 10, 20, 0.92); display: flex; align-items: center; justify-content: center; z-index: 200; }