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>

Cargo.toml

@@ -41,7 +41,7 @@ codec2 = "0.3"
 
 # Crypto
 x25519-dalek = { version = "2", features = ["static_secrets"] }
-ed25519-dalek = { version = "2", features = ["rand_core"] }
+ed25519-dalek = { version = "2", features = ["rand_core", "pkcs8"] }
 chacha20poly1305 = "0.10"
 hkdf = "0.12"
 sha2 = "0.10"