feat: export selection to WAV (band-limited, time-cropped)

New "Export WAV (W)" button + W key saves the current selection as a mono
WAV — the same band-limited, time-cropped audio you'd hear on playback, so
"what you hear is what you save". Self-documenting filename encodes the
time span and frequency band (rspektrum_sel_<t0>-<t1>s_<f0>-<f1>Hz.wav).

Refactor the shared filtered-region builder out of PlaySelectedRegion
(which also fixes a leak: it never freed regionSamples after
LoadSoundFromWave copies it). Make the export confirmation message persist
~3s instead of flashing for a single frame (affected PNG export too).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ui.c

@@ -323,6 +323,7 @@ void ExportPNG(const SpectrogramApp* spa, const char* dirPath)
     } else {
         snprintf((char*)spa->exportMessage, sizeof(spa->exportMessage), "Export failed");
     }
+    ((SpectrogramApp*)spa)->exportMessageTimer = 3.0f;
 
     RL_FREE(region.data);
 }
@@ -484,6 +485,15 @@ void DrawSidebar(void)
         DrawTextScaled("Export PNG (E)", exportButton.x + 10 * scale, exportButton.y + 7 * scale, 14, WHITE);
         y += 35 * scale;
 
+        // Export selection audio (band-limited, time-cropped) as a WAV
+        Rectangle wavButton = { x, y, sidebarWidth - 10 * scale, 30 * scale };
+        if (Clicked(wavButton)) {
+            ExportSelectionWAV(app.exportDir);
+        }
+        DrawPanelBox(wavButton, (Color){ 40, 60, 80, 255 }, CYAN);
+        DrawTextScaled("Export WAV (W)", wavButton.x + 10 * scale, wavButton.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 };