fix: pick bootloader based on firmware (BIOS → GRUB, UEFI → systemd-boot)

systemd-boot is UEFI-only. Hardcoding it broke the install on
BIOS/legacy hosts with HardwareIncompatibilityError in
installer._add_systemd_bootloader. Detect via /sys/firmware/efi and
fall back to GRUB for BIOS.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

webinstaller/app.py

@@ -450,12 +450,18 @@ def _post_install_commands(hostname):
     ]
 
 
+def _detect_bootloader():
+    # systemd-boot is UEFI-only; on BIOS/legacy it trips HardwareIncompatibilityError
+    # inside archinstall. /sys/firmware/efi exists iff we were booted via UEFI.
+    return "Systemd-boot" if Path("/sys/firmware/efi").exists() else "Grub"
+
+
 def build_archinstall_config(s):
     return {
         "archinstall-language": "English",
         "timezone": "Europe/Berlin",
         "ntp": True,
-        "bootloader": "Systemd-boot",
+        "bootloader": _detect_bootloader(),
         "disk_config": build_disk_config(s["boot_drive"]),
         "hostname": s["hostname"],
         "kernels": ["linux"],