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/app.js

@@ -449,6 +449,12 @@ $('#popover-overlay').addEventListener('click', (e) => {
     if (e.target === $('#popover-overlay')) closePopover();
 });
 
+$('#close-app-btn').addEventListener('click', async () => {
+    if (confirm('Close ItsGoin?\n\nStops all network connections to save battery. Reopen the app any time to resume.')) {
+        try { await invoke('exit_app'); } catch (_) {}
+    }
+});
+
 function relativeTime(timestampMs) {
     const now = Date.now();
     const diff = now - timestampMs;
@@ -4245,6 +4251,20 @@ $('#import-btn').addEventListener('click', () => {
     overlay.addEventListener('click', (e) => { if (e.target === overlay) overlay.remove(); });
 });
 
+(async () => {
+    const toggle = $('#session-relay-toggle');
+    if (!toggle) return;
+    try { toggle.checked = await invoke('get_session_relay_enabled'); } catch (_) {}
+    toggle.addEventListener('change', async () => {
+        try {
+            await invoke('set_session_relay_enabled', { enabled: toggle.checked });
+        } catch (e) {
+            toggle.checked = !toggle.checked;
+            alert('Failed to update session relay setting: ' + e);
+        }
+    });
+})();
+
 $('#notifications-btn').addEventListener('click', async () => {
     // Load current settings
     const msgVal = await invoke('get_setting', { key: 'notif_messages' }).catch(() => null) || 'on';