Refactor: extract platform abstraction for subprocess spawning

Move all Unix-specific syscalls (fork, execvp, waitpid, strerror, /tmp)
into a platform layer so the same spectrogram.c can target Windows
(CreateProcess) and WebAssembly (stubs). Key changes:

- src/platform.h: public API with spawn handle, error codes, path helpers
- src/platform_linux.c: fork/execvp implementation, /tmp, strerror
- src/platform_win32.c: CreateProcess implementation, temp dir lookup
- src/platform_web.c: stub implementations (no subprocess support on web)
- src/spectrogram.c: consume only platform.h; replace pid_t/fork/execvp
  with Platform_SpawnChild/WaitForChild; use Platform_GetTempDir() for
  /tmp path; replace strdup with malloc/memcpy for portability
- Build: add platform_*.c to gmake, Premake, and build_web.sh

src/spectrogram.c

@@ -1,11 +1,9 @@
 // spectrogram.c - Spectrogram viewer with region selection and playback
 // Based on Unity Audio-Experiments project
 
-#define _POSIX_C_SOURCE 200809L
-#define _DEFAULT_SOURCE
-
 #include "raylib.h"
 #include "resource_dir.h"
+#include "platform.h"
 
 #include <stdlib.h>
 #include <string.h>
@@ -418,21 +416,34 @@ static void FreeSTFT(StftResult* result)
 // Convert audio file to WAV using ffmpeg (if available)
 static bool ConvertToFFmpegWAV(const char* inputPath, char* outputPath, size_t outputSize)
 {
-    // Create temp WAV path
-    snprintf(outputPath, outputSize, "/tmp/rspektrum_temp_converted.wav");
-    
-    // Build ffmpeg command
-    char cmd[1024];
-    snprintf(cmd, sizeof(cmd), "ffmpeg -y -loglevel quiet -i \"%s\" -ar 48000 -ac 1 -f wav \"%s\" 2>&1", inputPath, outputPath);
-    
-    int result = system(cmd);
-    
-    if (result == 0 && FileExists(outputPath)) {
+    snprintf(outputPath, outputSize, "%s/rspektrum_temp_converted.wav",
+             Platform_GetTempDir());
+
+    /* Build argv array — no shell = no injection risk */
+    const char* argv[] = {
+        "ffmpeg", "-y", "-loglevel", "quiet",
+        "-i", inputPath,
+        "-ar", "48000", "-ac", "1", "-f", "wav",
+        outputPath, NULL
+    };
+
+    PlatformSpawnHandle handle = { 0 };
+    PlatformError rc = Platform_SpawnChild("ffmpeg", argv, &handle);
+    if (rc != PLATFORM_OK) {
+        TraceLog(LOG_WARNING, "Failed to spawn ffmpeg: %s", Platform_GetLastErrorMessage());
+        return false;
+    }
+
+    Platform_WaitForChild(&handle);
+
+    int exit_code = 0;
+    SpawnStatus status = Platform_GetExitStatus(&handle, &exit_code);
+    if (status == SPAWN_EXITED && exit_code == 0 && FileExists(outputPath)) {
         TraceLog(LOG_INFO, "FFmpeg conversion successful: %s", outputPath);
         return true;
     }
-    
-    TraceLog(LOG_WARNING, "FFmpeg conversion failed or not available");
+
+    TraceLog(LOG_WARNING, "FFmpeg conversion failed (exit code %d)", exit_code);
     return false;
 }
 
@@ -457,10 +468,10 @@ static bool LoadWavFile(const char* filepath, AudioSignal* signal)
     }
     
     Wave wave = LoadWave(filepath);
-    if (wave.data == NULL) { 
-        TraceLog(LOG_ERROR, "Failed to open WAV file: %s", filepath); 
-        if (isConverted) remove(convertedPath);
-        return false; 
+    if (wave.data == NULL) {
+        TraceLog(LOG_ERROR, "Failed to open WAV file: %s", filepath);
+        if (isConverted) FileRemove(convertedPath);
+        return false;
     }
     
     signal->sampleRate = wave.sampleRate;
@@ -493,7 +504,7 @@ static bool LoadWavFile(const char* filepath, AudioSignal* signal)
     
     // Clean up temp file if converted
     if (isConverted) {
-        remove(convertedPath);
+        FileRemove(convertedPath);
         TraceLog(LOG_INFO, "Cleaned up temp file: %s", convertedPath);
     }
     
@@ -768,9 +779,13 @@ static void ScanDirectory(const char* path)
             const char* name = GetFileName(files.paths[i]);
             if (strcmp(name, ".") == 0 || strcmp(name, "..") == 0) continue;
             if (DirectoryExists(files.paths[i])) {
-                app.browserFiles[app.browserFileCount] = strdup(name);
-                app.browserIsDir[app.browserFileCount] = true;
-                app.browserFileCount++;
+                size_t len = strlen(name) + 1;
+                app.browserFiles[app.browserFileCount] = (char*)malloc(len);
+                if (app.browserFiles[app.browserFileCount]) {
+                    memcpy(app.browserFiles[app.browserFileCount], name, len);
+                    app.browserIsDir[app.browserFileCount] = true;
+                    app.browserFileCount++;
+                }
             }
         }
         for (int i = 0; i < files.count; i++) {
@@ -780,9 +795,13 @@ static void ScanDirectory(const char* path)
                 const char* ext = GetFileExtension(files.paths[i]);
                 if (ext && (strcmp(ext, ".wav") == 0 || strcmp(ext, ".WAV") == 0 ||
                             strcmp(ext, ".Wave") == 0 || strcmp(ext, ".Wav") == 0)) {
-                    app.browserFiles[app.browserFileCount] = strdup(name);
-                    app.browserIsDir[app.browserFileCount] = false;
-                    app.browserFileCount++;
+                    size_t len = strlen(name) + 1;
+                    app.browserFiles[app.browserFileCount] = (char*)malloc(len);
+                    if (app.browserFiles[app.browserFileCount]) {
+                        memcpy(app.browserFiles[app.browserFileCount], name, len);
+                        app.browserIsDir[app.browserFileCount] = false;
+                        app.browserFileCount++;
+                    }
                 }
             }
         }