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/package.json

@@ -28,6 +28,9 @@
 		"vite": "^7.3.1"
 	},
 	"dependencies": {
+		"@xterm/addon-fit": "^0.11.0",
+		"@xterm/addon-web-links": "^0.12.0",
+		"@xterm/xterm": "^6.0.0",
 		"chart.js": "^4.5.1"
 	}
 }