Import sheet: give the source picker a hero header

Part of VAULT-77. This is the first screen of the import sheet: the root of BackupImportFlowView.

Now

• The screen has an inline "Import" title and no headline.
• It has two sections with grey headers, "Automatic Import" and "QR Code Import", and each holds a full-width ProminentActionButton ("Select PDF File" / "Start Scanning") with a footer.
• It looks the same whether you came from Import Backup, Import & Merge or Import & Override, so nothing says what's about to happen to the vault.
• Errors, such as a PDF that isn't a Vault backup, appear as a red PlaceholderView card above the buttons.

Change

• Add a BackupHeroHeader at the top, chosen by BackupImportContext. Use the same symbol as the Restore page row that opened the sheet, so the two screens read as one step:
  • .toEmptyVault: square.and.arrow.down, in the accent colour.
  • .merge: square.and.arrow.down.on.square, in the accent colour.
  • .override: exclamationmark.triangle.fill, in red. This screen should say, briefly, that everything on the device will be replaced.
  • Keep each subtitle to about one line: what to pick, and that you'll need the backup's password.
• Drop the "Automatic Import" / "QR Code Import" section headers. Either:
  • make the two sources rows with an icon, a title and a one-line explanation, like the successNote rows in BackupKeyChangeView; or
  • keep the two buttons in one section, without headers. Pick whichever reads better at xxLarge.
• Show errors in the header instead: swap the symbol for a red exclamationmark.triangle.fill (a symbol replace effect), use the error's title and description, and play an error haptic. When the next attempt starts, go back to the normal header.
• Keep the .snappy animations on importState / payloadState.

Done when

• The screen has a hero header that matches the Restore page row that opened it, for all three contexts.
• Errors appear in the header, with a haptic, and no card.
• BackupImportFlowViewSnapshotTests covers the three contexts plus an error state, and has been re-recorded at xSmall, medium and xxLarge in light and dark.