feat: add clipboard API for remote browser clipboard access

POST /api/clipboard with { text } broadcasts to all connected browsers
via SSE. Browser attempts navigator.clipboard.writeText, falling back
to a modal with manual copy button if blocked.

Enables remote clipboard workflows: CLI tools can push text to the
user's browser clipboard across the network.

src/web/server.ts

@@ -112,6 +112,7 @@ import {
   registerRespawnRoutes,
   registerRalphRoutes,
   registerPlanRoutes,
+  registerClipboardRoutes,
   registerOrchestratorRoutes,
   registerWsRoutes,
 } from './routes/index.js';
@@ -650,6 +651,7 @@ export class WebServer extends EventEmitter {
     registerRespawnRoutes(this.app, ctx);
     registerRalphRoutes(this.app, ctx);
     registerPlanRoutes(this.app, ctx);
+    registerClipboardRoutes(this.app, ctx);
     registerOrchestratorRoutes(this.app, ctx);
     registerWsRoutes(this.app, ctx);
   }