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/sw.js

@@ -0,0 +1,44 @@
+// Minimal service worker: only exists to catch the POST /share-target
+// request from Android's share sheet (required for Web Share Target to
+// work — the browser needs an installed SW controlling the scope). Stores
+// the shared files in a temporary Cache entry, then redirects to a page
+// that reads them and feeds them into the send flow.
+
+const SHARE_CACHE = 'flit-share-v1'
+
+self.addEventListener('install', (event) => {
+  self.skipWaiting()
+})
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(self.clients.claim())
+})
+
+self.addEventListener('fetch', (event) => {
+  const url = new URL(event.request.url)
+  if (event.request.method === 'POST' && url.pathname === '/share-target') {
+    event.respondWith(handleShareTarget(event))
+  }
+})
+
+async function handleShareTarget(event) {
+  const formData = await event.request.formData()
+  const files = formData.getAll('files')
+  const cache = await caches.open(SHARE_CACHE)
+  await cache.put('/shared-files', new Response(await packFiles(files)))
+  return Response.redirect('/?shared=1', 303)
+}
+
+async function packFiles(files) {
+  // Pack as a multipart-ish JSON manifest + blobs isn't trivial inside a SW;
+  // simplest robust approach: store each file as its own cache entry keyed
+  // by index, plus a manifest of names/types.
+  const cache = await caches.open(SHARE_CACHE)
+  const manifest = []
+  for (let i = 0; i < files.length; i++) {
+    const f = files[i]
+    manifest.push({ name: f.name, type: f.type, size: f.size, key: `/shared-file-${i}` })
+    await cache.put(`/shared-file-${i}`, new Response(f))
+  }
+  return JSON.stringify(manifest)
+}