Scaffold flit: PWA + Go CLI for ephemeral E2E file transfer

Ports the yaw/2.1 identity/signaling/WebRTC transport from waste-go to
both a browser PWA (with QR pairing and Web Share Target) and a headless
Go CLI, trimmed to 1:1 ephemeral file transfer only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pwa/public/manifest.json

@@ -0,0 +1,23 @@
+{
+  "name": "flit",
+  "short_name": "flit",
+  "description": "Ephemeral, end-to-end encrypted file transfer between your own devices.",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0d1117",
+  "theme_color": "#1f6feb",
+  "icons": [
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ],
+  "share_target": {
+    "action": "/share-target",
+    "method": "POST",
+    "enctype": "multipart/form-data",
+    "params": {
+      "files": [
+        { "name": "files", "accept": ["*/*"] }
+      ]
+    }
+  }
+}