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/public/app.js

@@ -249,6 +249,9 @@ const _SSE_HANDLER_MAP = [
   [SSE_EVENTS.ORCHESTRATOR_TASK_FAILED, '_onOrchestratorTaskFailed'],
   [SSE_EVENTS.ORCHESTRATOR_COMPLETED, '_onOrchestratorCompleted'],
   [SSE_EVENTS.ORCHESTRATOR_ERROR, '_onOrchestratorError'],
+
+  // Clipboard
+  [SSE_EVENTS.CLIPBOARD_WRITE, '_onClipboardWrite'],
 ];