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/handler/multiplex.go

@@ -25,7 +25,11 @@ func NewMultiplexedChannel[T any](buffer int) *MultiplexedChannel[T] {
 			c.mu.RLock()
 
 			for _, cons := range c.channels {
-				cons <- v
+				select {
+				case cons <- v:
+				default:
+					// Consumer not reading — skip to prevent deadlock
+				}
 			}
 
 			c.mu.RUnlock()
@@ -52,7 +56,7 @@ func (m *MultiplexedChannel[T]) Fork() (chan T, func()) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
-	consumer := make(chan T)
+	consumer := make(chan T, 64)
 
 	m.channels = append(m.channels, consumer)