init: FileDrop phase1 architecture and scaffold

- Rust axum signaling server with WebSocket support
- Lit + TypeScript frontend with Vite
- Redis session storage with TTL
- WebRTC transport and crypto client stubs
- Phase1 architecture plan in plans/
- Deploy directory structure prepared

web/src/webrtc/transport.ts

@@ -0,0 +1,70 @@
+export class WebRTCTransport {
+  private pc: RTCPeerConnection | null = null;
+  private dc: RTCDataChannel | null = null;
+  private iceServers: RTCIceServer[];
+
+  constructor(iceServers: RTCIceServer[]) {
+    this.iceServers = iceServers;
+  }
+
+  async createOffer(): Promise<{ sdp: string; candidates: RTCIceCandidate[] }> {
+    this.pc = new RTCPeerConnection({ iceServers: this.iceServers });
+    this.dc = this.pc.createDataChannel('filedrop', { ordered: true });
+
+    const offer = await this.pc.createOffer();
+    await this.pc.setLocalDescription(offer);
+
+    const candidates = await this.waitForIceGathering();
+    return {
+      sdp: this.pc.localDescription?.sdp || '',
+      candidates,
+    };
+  }
+
+  async handleAnswer(answer: { sdp: string }) {
+    if (!this.pc) return;
+    await this.pc.setRemoteDescription(
+      new RTCSessionDescription({ type: 'answer', sdp: answer.sdp })
+    );
+  }
+
+  async handleCandidate(candidate: RTCIceCandidateInit) {
+    await this.pc?.addIceCandidate(candidate);
+  }
+
+  onDataChannel(callback: (dc: RTCDataChannel) => void) {
+    if (this.pc) {
+      this.pc.ondatachannel = (e) => callback(e.channel);
+    }
+  }
+
+  getDataChannel() {
+    return this.dc;
+  }
+
+  private waitForIceGathering(): Promise<RTCIceCandidate[]> {
+    return new Promise((resolve) => {
+      const candidates: RTCIceCandidate[] = [];
+      if (!this.pc) return resolve(candidates);
+
+      this.pc.onicecandidate = (e) => {
+        if (e.candidate) candidates.push(e.candidate);
+      };
+
+      this.pc.onicegatheringstatechange = () => {
+        if (this.pc?.iceGatheringState === 'complete') {
+          resolve(candidates);
+        }
+      };
+
+      setTimeout(() => resolve(candidates), 5000);
+    });
+  }
+
+  close() {
+    this.dc?.close();
+    this.pc?.close();
+    this.dc = null;
+    this.pc = null;
+  }
+}