feat: add PNG export for spectrogram image

Add Export PNG button and keyboard shortcut (E) to export the
spectrogramImage as a PNG file. Supports:
  - Cropping to the current time/frequency selection region
  - Optional upscaling via the export scale slider (0.0x to 10.0x)
  - Exporting the full unscaled image when no selection and scale is 0

Exports to the app's working directory via raylib's ExportImage.
Export status shown as a brief toast notification.

Known issues:
  - Frequency scale doesn't adjust properly when zooming (freqView
    clamps to [0,1] range but zoom factor isn't frequency-aware)
  - Hardcoded export filenames; no filename/path picker yet

src/spectrogram.c

@@ -111,6 +111,12 @@ typedef struct {
     float freqSelectionStart;
     float freqSelectionEnd;
     bool isFreqSelecting;
+
+    // Export settings
+    float exportScale;
+    char exportDir[4096];
+    char exportMessage[256];
+
     Vector2 selectStartPos;  // For minimum drag distance check
     bool isDraggingSelection;  // Dragging existing selection box
     Vector2 dragSelectionStartPos;  // Mouse position when started dragging selection
@@ -1209,6 +1215,79 @@ static void DrawSelectionDrag(Rectangle bounds)
     DrawRectangleLinesEx((Rectangle){ x, y, w, h }, 2, YELLOW);
 }
 
+// ============================================================================
+// PNG Export
+// ============================================================================
+
+static void ExportPNG(const SpectrogramApp* spa, const char* dirPath)
+{
+    if (!spa->stftComputed || !spa->spectrogramImage.data) return;
+
+    int imgW = spa->spectrogramImage.width;
+    int imgH = spa->spectrogramImage.height;
+
+    // Selection region in image-pixel coordinates
+    int selX0 = (int)(spa->timeSelectionStart * imgW);
+    int selX1 = (int)(spa->timeSelectionEnd   * imgW);
+    int selY0 = (int)((1.0f - spa->freqSelectionEnd) * imgH);
+    int selY1 = (int)((1.0f - spa->freqSelectionStart) * imgH);
+
+    // Clamp to image bounds
+    selX0 = Clamp(selX0, 0, imgW);
+    selX1 = Clamp(selX1, 0, imgW);
+    selY0 = Clamp(selY0, 0, imgH);
+    selY1 = Clamp(selY1, 0, imgH);
+
+    int regionW = selX1 - selX0;
+    int regionH = selY1 - selY0;
+    if (regionW <= 0 || regionH <= 0) return;
+
+    // Extract selected region by copying pixel rows
+    Image region = { 0 };
+    region.width  = regionW;
+    region.height = regionH;
+    region.mipmaps = 1;
+    region.format = spa->spectrogramImage.format;
+    region.data = RL_MALLOC(regionW * regionH * 4);
+    if (!region.data) return;
+
+    // Raylib stores images as tightly packed RGBA rows - memcpy each row at a time
+    Color* src = (Color*)spa->spectrogramImage.data;
+    Color* dst = (Color*)region.data;
+    for (int y = 0; y < regionH; y++) {
+        memcpy(dst + y * regionW,
+               src + (selY0 + y) * imgW + selX0,
+               regionW * 4);
+    }
+
+    // Scale if a non-zero exportScale is set (and region isn't too large)
+    if (spa->exportScale > 0.0f && regionW < 4096) {
+        int outW = regionW * (int)spa->exportScale;
+        int outH = regionH * (int)spa->exportScale;
+        if (outW > 0 && outH > 0) {
+            ImageResize(&region, outW, outH);
+        }
+    }
+
+    // Export as PNG
+    char path[4096];
+    if (spa->exportScale <= 0.0f && regionW == imgW) {
+        // Full width export without scaling
+        snprintf(path, sizeof(path), "%s/spectrogram_full.png", dirPath);
+    } else {
+        snprintf(path, sizeof(path), "%s/spectrogram_export.png", dirPath);
+    }
+
+    if (ExportImage(region, path)) {
+        snprintf((char*)spa->exportMessage, sizeof(spa->exportMessage),
+                 "Exported: %dx%d %.40s", region.width, region.height, path);
+    } else {
+        snprintf((char*)spa->exportMessage, sizeof(spa->exportMessage), "Export failed");
+    }
+
+    RL_FREE(region.data);
+}
+
 static void DrawSidebar(void)
 {
     float scale = GetUIScale();
@@ -1360,6 +1439,28 @@ static void DrawSidebar(void)
     DrawTextScaled("Clear (ESC)", clearButton.x + 10 * scale, clearButton.y + 6 * scale, 14, WHITE);
     y += 38 * scale;
 
+    // Export PNG
+    {
+        Rectangle exportButton = { x, y, sidebarWidth - 10 * scale, 30 * scale };
+        if (CheckCollisionPointRec(GetMousePosition(), exportButton) && IsMouseButtonPressed(MOUSE_LEFT_BUTTON)) {
+            ExportPNG(&app, app.exportDir);
+        }
+        DrawRectangleRec(exportButton, (Color){ 40, 60, 80, 255 });
+        DrawRectangleLinesEx(exportButton, 1, CYAN);
+        DrawTextScaled("Export PNG (E)", exportButton.x + 10 * scale, exportButton.y + 7 * scale, 14, WHITE);
+        y += 35 * scale;
+
+        // Export scale slider
+        DrawTextScaled(TextFormat("Export Scale: %.1fx", app.exportScale), x, y, 12, LIGHTGRAY); y += 18 * scale;
+        Rectangle scaleSlider = { x, y, sidebarWidth - 10 * scale, 14 * scale };
+        DrawSlider(scaleSlider, app.exportScale / 10.0f);
+        float scaleVal = 0.0f;
+        if (UpdateSlider(scaleSlider, &scaleVal)) {
+            app.exportScale = scaleVal * 10.0f;
+        }
+        y += 24 * scale;
+    }
+
     // Signal info
     DrawTextScaled("Signal Info:", x, y, 14, LIGHTGRAY); y += 20 * scale;
     if (app.loaded) {
@@ -1420,10 +1521,15 @@ int main(int argc, char* argv[])
 
     // Save original working directory so command-line args resolve correctly
     // before we change working dir to resources/
-    static char originalDir[512] = { 0 };
+    static char originalDir[4096] = { 0 };
     snprintf(originalDir, sizeof(originalDir), "%s", GetWorkingDirectory());
     TraceLog(LOG_INFO, "Original working directory: %s", originalDir);
 
+    // Set export directory to the app's working directory (before CWD changes)
+    snprintf(app.exportDir, sizeof(app.exportDir), "%s", originalDir);
+    app.exportScale = 1.0f;
+    app.exportMessage[0] = '\0';
+
     SearchAndSetResourceDir("resources");
 
     // Load TTF font at a fixed base size. Scaling is handled uniformly by
@@ -1464,7 +1570,7 @@ int main(int argc, char* argv[])
     bool fileLoaded = false;
     if (argc > 1) {
         TraceLog(LOG_INFO, "Loading file from command line: %s", argv[1]);
-        char resolvedPath[4096] = { 0 };
+        char resolvedPath[8192] = { 0 };
 
         // If the path doesn't exist as-is, try prepending original dir
         if (!FileExists(argv[1]) && originalDir[0]) {
@@ -1700,6 +1806,11 @@ int main(int argc, char* argv[])
             }
         }
 
+        // Export PNG
+        if (IsKeyPressed(KEY_E) && !app.showFileBrowser && app.stftComputed) {
+            ExportPNG(&app, app.exportDir);
+        }
+
         if (IsKeyPressed(KEY_ESCAPE)) {
             if (app.showFileBrowser) {
                 app.showFileBrowser = false;
@@ -2097,6 +2208,21 @@ int main(int argc, char* argv[])
         // Draw file browser on top (if active)
         if (app.showFileBrowser) DrawFileBrowser();
 
+        // Export message notification
+        if (app.exportMessage[0] != '\0') {
+            int msgW = MeasureText(app.exportMessage, 20);
+            int boxW = msgW + 40;
+            int boxH = 36;
+            int boxX = GetScreenWidth() / 2 - boxW / 2;
+            int boxY = 15;
+            DrawRectangle(boxX, boxY, boxW, boxH, (Color){ 30, 30, 30, 220 });
+            DrawRectangleLines(boxX, boxY, boxW, boxH, CYAN);
+            DrawText(app.exportMessage, boxX + (boxW - msgW) / 2, boxY + 10, 20, WHITE);
+        }
+
+        // Reset export message after one frame display
+        app.exportMessage[0] = '\0';
+
         EndDrawing();
     }