fix(webinstaller): FilesystemType.Ext4 → .EXT4 for newer archinstall

archinstall's enum members got renamed from CapitalCase (Ext4) to
ALL_CAPS (EXT4) between when build_disk_config was written and the
version baked into the current Arch live ISO. Result: the install
step fires AttributeError at the archinstall JSON-dump moment,
rendering a Flask 500 right after the Confirm page.

Traceback from the VM:
  File "/opt/furtka/app.py", line 128, in build_disk_config
    filesystem_type=FilesystemType.Ext4,
AttributeError: type object 'FilesystemType' has no attribute
  'Ext4'. Did you mean: 'EXT4'?

Tests pass because build_disk_config is monkeypatched out in CI
(archinstall isn't pip-installable; it only exists on the live ISO).

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

webinstaller/app.py

@@ -125,7 +125,10 @@ def build_disk_config(boot_drive):
     device_mod = asyncio.run(
         suggest_single_disk_layout(
             device,
-            filesystem_type=FilesystemType.Ext4,
+            # archinstall renamed the enum members to ALL_CAPS at some point
+            # between when we wrote this and the pinned Arch live ISO version.
+            # The old name `Ext4` now raises AttributeError at install time.
+            filesystem_type=FilesystemType.EXT4,
             separate_home=False,
         )
     )