Add configuration macros for default run mode

Add simple macros to configure default behavior:
- DEFAULT_CONFIG_FILE: which config to load
- DEFAULT_HEADLESS: GUI or headless mode
- DEFAULT_READABLE: scientific or human-readable output
- DEFAULT_SIM_DAYS: simulation duration

Command-line arguments override these defaults.

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/main.cpp

@@ -7,7 +7,13 @@
 #include <cstdlib>
 #include <cmath>
 
-// Constants for unit conversion
+// Configuration defaults - edit to change default run mode
+#define DEFAULT_CONFIG_FILE "configs/solar_system.txt"
+#define DEFAULT_HEADLESS 0      // 0 = GUI mode, 1 = headless mode
+#define DEFAULT_READABLE 0      // 0 = scientific notation, 1 = human-readable (AU, km/s)
+#define DEFAULT_SIM_DAYS 365    // Default simulation duration for headless mode
+
+// Unit conversion constants
 const double AU = 1.496e11;  // 1 AU in meters
 const double KM = 1000.0;     // 1 km in meters
 
@@ -41,11 +47,11 @@ void print_body_readable(CelestialBody* body) {
 }
 
 int main(int argc, char** argv) {
-    // Parse command line arguments
-    const char* config_file = "configs/solar_system.txt";
-    bool headless = false;
-    bool readable = false;  // Human-readable output (AU, km, etc.)
-    int sim_duration_days = 365;  // Default: 1 year for headless mode
+    // Parse command line arguments (start with macro defaults)
+    const char* config_file = DEFAULT_CONFIG_FILE;
+    bool headless = DEFAULT_HEADLESS;
+    bool readable = DEFAULT_READABLE;
+    int sim_duration_days = DEFAULT_SIM_DAYS;
 
     for (int i = 1; i < argc; i++) {
         if (strcmp(argv[i], "--headless") == 0 || strcmp(argv[i], "-h") == 0) {