Lock a gift to a nostr key: public link, claim code DM'd to the recipient

A gift can now be locked to a recipient's nostr npub. The shareable link
(/lg/<blob>) carries the amount + recipient in the clear (anyone can see what it
is and who it's for), but the claim payload is encrypted under a one-time code.
That code is delivered as an encrypted nostr DM (NIP-04) to the recipient — so
only they can read it. They paste the code to decrypt, then claim into a fresh
wallet exactly like a normal bearer gift.

- nostr.js: parseNostrPubkey/npubOf, encryptWithCode/decryptWithCode (random
  32-byte code), fetchNostrProfile, NostrSync.sendDM (broad relays).
- wallet.js: lockGift → { blob, claimCode }, previewLockedGift, nostr identity +
  sendNostrDM.
- app.js: gift form gains an optional 'Lock to npub' field that shows the
  recipient's avatar + name (nostr kind-0 profile); create encrypts + DMs the
  code (with sent/failed status + a copyable fallback); /lg/ links open a claim
  view showing the gift + recipient where you paste the code; account settings
  shows your own npub to share.

Verified: code lock/unlock round-trip, a real nostr DM published and read back
from relay.damus.io, and the UI loads + renders with no errors.

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

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