fix(desktop): show Continue when a raw nsec is loaded in the backup restore dialog

[BradGroux]

Note: This is a replacement for PR #5308, which was accidentally closed when branches were force-pushed after a commit identity rewrite. The changes are identical, rebased onto the latest main.

Summary

The backup-file restore dialog accepts .key files containing a raw nsec1 private key. The file parser loads the key and displays "Nostr identity found" with the derived npub, but the submit button only rendered when mode === "key" or isPasswordStage (ncryptsec). A raw nsec in mode="backup" satisfies neither condition, so the user sees a recognized identity with no Continue button — a dead end with no forward action.

Add isValid to the submit button's render guard so a valid raw nsec shows the Continue button regardless of mode. The button's existing disabled={!isValid} check prevents submission of invalid input, and keyImportSubmitEnabled already validates the nsec by deriving its npub.

Related issue

Fixes #5261.

Testing

• Desktop Biome, file-size, text-size, and pubkey-truncation checks passed.
• TypeScript typecheck and production Vite build passed.
• Full Tauri test suite passed: 2,270 tests, 0 failures, 14 ignored.
• Desktop JS test suite passed.

[BradGroux]

This is a replacement for #5308, which was accidentally closed when branches were force-pushed after a commit identity rewrite. Prior review comments and feedback are preserved on the old PR:

#5308