fix(frontend): show save button when mount type is disabled

[javi11]

Summary

Fixes #611 — selecting the Disabled mount type left the user with no
Save button, so the change could not be persisted through the UI (users
had to hand-edit config.yaml or call the REST API).

Root cause

In MountConfigSection.tsx
the entire Save block was wrapped in {mountType !== "none" && (...)}.
Once "Disabled" was selected the block (and the button) disappeared.
The button was also disabled while !mountPath, which has no meaning
when the mount is disabled.

Change

• Render the Save block unconditionally.
• Keep the "Regenerate symlinks" checkbox gated on mountType !== "none" && mountPathChanged (it only applies to an active mount).
• Relax the disabled check to (mountType !== "none" && !mountPath) so an empty path no longer blocks saving the disabled state.

No backend change needed — mount_type: "none" is already accepted, and the
success toast already produces "Mount type set to disabled".

Test plan

• cd frontend && bun run build succeeds.
• Start the app, open Configuration → Mount.
• With an active mount (e.g. fuse or rclone) selected, click Disabled, confirm the dialog, and verify the Save button now appears and is enabled.
• Click Save and verify the success toast ("Mount type set to disabled") and that the disabled state persists across reload.
• With mount already disabled and no pending changes, verify the Save button is visible but disabled.
• Switch from Disabled back to an active type and verify the existing behavior (sub-sections appear, mount path required) is unchanged.