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>

src/offers.ts

@@ -195,23 +195,6 @@ export class RondevuOffers {
     return data.offers;
   }
 
-  /**
-   * Update offer heartbeat
-   */
-  async heartbeat(offerId: string): Promise<void> {
-    const response = await this.fetchFn(`${this.baseUrl}/offers/${encodeURIComponent(offerId)}/heartbeat`, {
-      method: 'PUT',
-      headers: {
-        Authorization: RondevuAuth.createAuthHeader(this.credentials),
-      },
-    });
-
-    if (!response.ok) {
-      const error = await response.json().catch(() => ({ error: 'Unknown error' }));
-      throw new Error(`Failed to update heartbeat: ${error.error || response.statusText}`);
-    }
-  }
-
   /**
    * Delete an offer
    */