Add terminal tab to capsule detail page and fix envd process lookup bugs

- Add multi-session Terminal tab with xterm.js (session tabs, close, reconnect)
- Keep terminal mounted across tab switches to preserve sessions
- Persist active tab in URL (?tab=terminal) so refresh stays on terminal
- Buffer keystrokes (50ms) to reduce per-character RPC overhead
- Add WebSocket auth via ?token= query param for browser WS connections
- Enable ws:true in Vite dev proxy for WebSocket support

envd fixes (pre-existing bugs exposed by multi-session terminals):
- Fix getProcess tag Range: inverted return values caused early stop when
  multiple tagged processes existed, making SendInput fail with "not found"
- Fix multiplexer deadlock: blocking send to cancelled fork's unbuffered
  channel prevented process cleanup. Now uses buffered channels (cap 64)
  with non-blocking fallback

envd/internal/services/process/service.go

@@ -62,16 +62,15 @@ func (s *Service) getProcess(selector *rpc.ProcessSelector) (*handler.Handler, e
 
 		s.processes.Range(func(_ uint32, value *handler.Handler) bool {
 			if value.Tag == nil {
-				return true
+				return true // no tag, keep looking
 			}
 
 			if *value.Tag == tag {
 				proc = value
-
-				return true
+				return false // found, stop iterating
 			}
 
-			return false
+			return true // different tag, keep looking
 		})
 
 		if proc == nil {