feat: migrate to system dependencies from static linking

BREAKING CHANGE: Library now requires system-installed dependencies

Major Changes:
- Convert secp256k1 from bundled static lib to system dependency
- Convert OpenSSL from bundled static lib to system dependency
- Convert curl from bundled static lib to system dependency
- Update build.sh with pkg-config detection and fallback logic
- Remove all static library extraction/building logic
- Update README.md with new dependency requirements and installation

Build System:
- Add detect_system_secp256k1() with pkg-config support
- Add detect_system_openssl() with pkg-config support
- Add detect_system_curl() with pkg-config support
- Remove secp256k1 building/extraction from ar archive
- Update CFLAGS and LIBS to use system library variables
- Clear error messages for missing dependencies with install commands

Documentation:
- Add system dependency installation for Ubuntu/Debian/CentOS/macOS
- Update all compile/link examples to include -lssl -lcrypto -lcurl -lsecp256k1
- Remove references to 'self-contained' and 'no external dependencies'
- Update integration examples throughout README

Benefits:
- Smaller library size (only internal code bundled)
- Automatic security updates via system package manager
- Standard Linux library distribution pattern
- Reduced build complexity
- Better system integration with pkg-config

Required Installation:
Ubuntu/Debian: sudo apt install libssl-dev libcurl4-openssl-dev libsecp256k1-dev
CentOS/RHEL: sudo yum install openssl-devel libcurl-devel libsecp256k1-devel
macOS: brew install openssl curl secp256k1

test_crypto_debug.c

@@ -0,0 +1,33 @@
+#include "nip004.h"
+#include "utils.h"
+#include <stdio.h>
+
+int main() {
+    printf("Testing crypto initialization...\n");
+    
+    if (nostr_crypto_init() != 0) {
+        printf("❌ Crypto init failed\n");
+        return 1;
+    }
+    printf("✅ Crypto init successful\n");
+    
+    // Test random bytes generation
+    unsigned char random_bytes[16];
+    if (nostr_secp256k1_get_random_bytes(random_bytes, 16) != 1) {
+        printf("❌ Random bytes generation failed\n");
+        return 1;
+    }
+    printf("✅ Random bytes generation successful\n");
+    
+    // Test base64 encoding
+    char output[64];
+    size_t encoded_len = base64_encode(random_bytes, 16, output, sizeof(output));
+    if (encoded_len == 0) {
+        printf("❌ Base64 encoding failed\n");
+        return 1;
+    }
+    printf("✅ Base64 encoding successful: %s\n", output);
+    
+    nostr_crypto_cleanup();
+    return 0;
+}