Vault primary buttons no longer look disabled in dark mode

Fixed

• Vault buttons (Create / Unlock / Save) now read as active in dark mode. Their filled label used --bg-surface, which is dark in dark mode and sat on the medium-grey dark accent at ~2.2:1 contrast, so an enabled button looked disabled. Switched to #fff to match every other accent-filled button in the app. Light mode is unchanged (the label was already white there).