Fold watch-only into Import; support xprv spending import

Home screen is back to two tabs (Create / Import). The Import box now
takes a recovery phrase, an xpub/zpub, or an xprv/zprv:
- mnemonic → full wallet (as before)
- xpub/zpub → watch-only (view/receive, no Send)
- xprv/zprv → full spending wallet with no mnemonic. parseExtendedKey
  classifies + normalizes version bytes; account() derives the BIP84
  path from a master xprv (depth 0) or uses an account-level xprv as-is;
  signing works (only watch-only blocks it). Vault persists xprv accounts
  too; Settings shows an "Imported key" note instead of a recovery card
  and hides Nostr sync (no seed). Public-key export still works.

Note: a seed phrase cannot be derived from an xprv (one-way KDF), so an
xprv wallet has no recovery words by design. New strings in all 19 locales.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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