fix: new-file load resets full nav state (zoom out both axes, stop playback)

ResetForNewSignal only reset the time view + selection, leaving the
previous file's frequency zoom, in-progress drags, and playback running.
Loading a new file now zooms out both axes, cancels any drag/pan/divide,
clears the selection, and stops playback + rewinds the playhead. Display
preferences (colormap, dB scale, FFT size, grid, scope layout) are
preserved by design. All three load paths funnel through this function.

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

src/spectrogram.c

@@ -118,9 +118,24 @@ void ResetForNewSignal(void)
     // Cached STFT results are tied to the old signal data.
     FreeAllCacheEntries(&app.fftCache);
 
-    // Reset view + selection to full range.
+    // Zoom out both axes and drop the old selection / any in-progress drags.
+    // Display preferences (colormap, dB scale, FFT size, grid, scope layout)
+    // are intentionally preserved across loads.
     app.view.start = 0.0f; app.view.end = 1.0f;
+    app.view.freqStart = 0.0f; app.view.freqEnd = 1.0f;
+    app.view.isPanning = false;
     ClearSelection();
+    app.sel.isDragging = false;
+    app.sel.isTimeSelecting = false;
+    app.sel.isFreqSelecting = false;
+    app.isDividing = false;
+
+    // Stop any playback from the previous signal and rewind the playhead.
+    if (app.isPlaying && AudioPlaybackSound.frameCount > 0) StopSound(AudioPlaybackSound);
+    app.isPlaying = false;
+    app.playbackFinished = false;
+    app.playheadElapsed = 0.0f;
+    app.playheadT = 0.0f;
 
     // Invalidate the cached visible texture.
     if (app.visibleTexture.id != 0) UnloadTexture(app.visibleTexture);