feat: direct calling UI for desktop Tauri app + merge android branch

Tauri backend:
- register_signal: persistent _signal connection, presence registration
- place_call: send DirectCallOffer by fingerprint
- answer_call: accept/reject incoming calls
- get_signal_status: poll signal state

Frontend:
- Mode toggle: "Room" vs "Direct Call"
- Register button → registers on relay signal channel
- Incoming call panel with Accept/Reject
- Fingerprint input + Call button
- Auto-connect to media room on CallSetup event

Also merges feat/android-voip-client into desktop branch:
- Federation fixes, time-based dedup, FEC stale blocks
- Direct calling protocol types
- ACL + SAS verification

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src/style.css

@@ -870,3 +870,23 @@ button.primary:disabled { opacity: 0.5; cursor: not-allowed; }
 .settings-section select:focus {
   border-color: var(--accent);
 }
+
+/* Direct calling mode toggle */
+.mode-btn {
+  padding: 8px 16px;
+  border: 1px solid var(--surface2);
+  background: var(--surface);
+  color: var(--dim);
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: 13px;
+  transition: all 0.15s;
+}
+.mode-btn.active {
+  background: var(--accent);
+  color: white;
+  border-color: var(--accent);
+}
+.mode-btn:hover:not(.active) {
+  background: var(--surface2);
+}