feat: LAN Wi-Fi casting to Chromecast via local HTTP relay

Re-serve decrypted fMP4 video over HTTP on the LAN so a Chromecast can
play the stream. The relay runs in the Electron main process — same trust
boundary as downloads and MSE playback.

- cast-relay.js: HTTP server with BoxAccumulator (reassembles WebRTC
  chunks into moof+mdat pairs), ring buffer, backpressure, finish() for
  clean end-of-stream, fixed port range 19550-19553
- cast-chromecast.js: mDNS discovery (bonjour-service) + CASTV2 protocol
  (castv2-client), connect/reload/disconnect lifecycle
- Seek-aware: relay restarts on every seek, Chromecast reloads new URL;
  generation counter prevents stale async errors from killing active
  restarts; landingPlayheadRef suppresses programmatic seeking events
- Device picker in video top bar with scan, device selection, copy-URL
  fallback, and cast status indicator
- IPC bridge (main/preload/platform) for start/push/stop/finish/status/
  discover/chromecastConnect/chromecastReload/chromecastDisconnect
- Phase 3 design doc for DLNA/Smart TV in docs/cast-smart-tv.md

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

1 files changed, 16 insertions, 0 deletions

diff --git a/packages/meshbay-client/src/preload.js b/packages/meshbay-client/src/preload.js
index 04dfe44..e9fd375 100644
--- a/packages/meshbay-client/src/preload.js
+++ b/packages/meshbay-client/src/preload.js
@@ -40,6 +40,7 @@ contextBridge.exposeInMainWorld('meshbay', {
     nodeAdmin: true,
     localFolders: true,
     nativeSave: true,
+    lanCast: true,
   },
 
   // Ask the main process to call the hub. The renderer has an `app://` origin,
@@ -90,6 +91,21 @@ contextBridge.exposeInMainWorld('meshbay', {
     setPairingCode: (code) => ipcRenderer.invoke('node:set-pairing-code', code),
   },
 
+  // LAN cast relay. The main process runs a local HTTP server and the
+  // renderer feeds it decrypted segments. A Chromecast or Smart TV on the
+  // same Wi-Fi plays from the URL.
+  cast: {
+    start: (opts) => ipcRenderer.invoke('cast:start', opts),
+    push: (data) => ipcRenderer.invoke('cast:push', data),
+    stop: () => ipcRenderer.invoke('cast:stop'),
+    finish: () => ipcRenderer.invoke('cast:finish'),
+    status: () => ipcRenderer.invoke('cast:status'),
+    discover: () => ipcRenderer.invoke('cast:discover'),
+    chromecastConnect: (opts) => ipcRenderer.invoke('cast:chromecast:connect', opts),
+    chromecastReload: (opts) => ipcRenderer.invoke('cast:chromecast:reload', opts),
+    chromecastDisconnect: () => ipcRenderer.invoke('cast:chromecast:disconnect'),
+  },
+
   // A sink that writes to disk as chunks arrive, never a buffer handed over at
   // the end. `auto` uses the remembered folder without a dialog, which is what
   // "save automatically" means; without one, or when the person asked to be