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

frontend/vite.config.ts

@@ -8,7 +8,8 @@ export default defineConfig({
 		proxy: {
 			'/api': {
 				target: 'http://localhost:8080',
-				rewrite: (path) => path.replace(/^\/api/, '')
+				rewrite: (path) => path.replace(/^\/api/, ''),
+				ws: true
 			}
 		}
 	}