Fix: Remove compiler warnings (signedness, unused functions, snprintf truncation)

- Cast st_size to uint64_t to match file_size type in server.c
- Remove unused functions: send_one_file, send_file_udp, verify_and_resend
- Reduce rel buffer size to 1024 to prevent snprintf truncation in collect_files

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/server.c

@@ -353,7 +353,7 @@ void handle_client(int sock_fd, const char *dest_dir) {
                 // Check if file already exists with same size and checksum (skip if so)
                 struct stat existing_st;
                 if (stat(target_path, &existing_st) == 0 && 
-                    existing_st.st_size == meta.file_size &&
+                    (uint64_t)existing_st.st_size == meta.file_size &&
                     S_ISREG(existing_st.st_mode)) {
                     // File exists with same size - check checksum
                     uint64_t existing_checksum = fast_hash_file(target_path, meta.file_size);