Add peer-to-peer file transfer in browser mode

Implements both pull (browse & download from a shared folder) and push
(send a file directly to a peer) using the yaw2 file protocol over WebRTC
DataChannels.

- PeerConn: handle browse/files/get/file-offer/file-accept/file-done
  control messages; stream files in 64KB chunks over f:xid binary
  DataChannels; auto-accept incoming offers (push and pull)
- PeerConn.sendFilePush(): initiate an outbound file transfer without
  requiring the peer to browse first
- BrowserAdapter: sharedFiles map, setSharedFiles(), requestBrowse(),
  requestGet(), sendFileTo() — propagates share map to all active peers
- Store: activeFilePeer, setActiveFilePeer, setSharedFiles, browseFiles,
  sendFileTo; file_complete handler triggers browser download automatically
- FileBrowser component: third-column panel listing a peer's shared files
  with name, size, and a download button
- FolderPicker component: webkitdirectory input in the sidebar (browser
  mode only) for selecting a local folder to share
- Sidebar: 📎 send-file button on peer rows (hover) with inline file picker
- App.tsx: use browser mode when WASTE_CONFIG.signalURL is set, even on
  localhost — allows testing browser mode via npm run dev with config.js
- deploy-daemon.sh: kill existing daemon before scp to avoid upload failure
  when the binary is locked; remove duplicate kill block in restart step

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

web/src/pages/Chat.tsx

@@ -1,11 +1,15 @@
 import { Sidebar } from '../components/Sidebar'
 import { MessagePane } from '../components/MessagePane'
+import { FileBrowser } from '../components/FileBrowser'
+import { useWaste } from '../store'
 
 export function Chat() {
+  const { activeFilePeer } = useWaste()
   return (
-    <div className="chat-layout">
+    <div className={`chat-layout${activeFilePeer ? ' has-file-browser' : ''}`}>
       <Sidebar />
       <MessagePane />
+      {activeFilePeer && <FileBrowser />}
     </div>
   )
 }