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/constants.js

@@ -292,6 +292,9 @@ const SSE_EVENTS = {
   ORCHESTRATOR_COMPLETED: 'orchestrator:completed',
   ORCHESTRATOR_ERROR: 'orchestrator:error',
 
+  // Clipboard
+  CLIPBOARD_WRITE: 'clipboard:write',
+
   // Cases
   CASE_CREATED: 'case:created',
   CASE_LINKED: 'case:linked',