fix: prevent [No Host] panel errors behind CDN or missing Host header

Force nginx to pass the domain as Host, add PANEL_DOMAIN fallback in Flask,
and document that the admin panel must be accessed over HTTP not HTTPS.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/install.sh

@@ -216,6 +216,7 @@ Type=simple
 WorkingDirectory=${ROOT_DIR}/panel
 Environment=JIEDIAN_ROOT=${ROOT_DIR}
 Environment=PANEL_PATH=${PANEL_PATH}
+Environment=PANEL_DOMAIN=${DOMAIN}
 ExecStart=${ROOT_DIR}/panel/venv/bin/python app.py
 Restart=on-failure
 RestartSec=5