Remove unused code (legacy files and heartbeat method)

- Removed unused legacy files: client.ts and types.ts (old API)
- Removed heartbeat() method from offers API (doesn't actually reset TTL)
- Removed heartbeat() documentation from README
- Server only uses expires_at for cleanup, last_seen is never checked
- Offers expire purely based on their original TTL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -418,13 +418,6 @@ Get all offers owned by the authenticated peer.
 const myOffers = await client.offers.getMine();
 ```
 
-#### `client.offers.heartbeat(offerId)`
-Update last_seen timestamp for an offer.
-
-```typescript
-await client.offers.heartbeat(offerId);
-```
-
 #### `client.offers.delete(offerId)`
 Delete a specific offer.