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/spectrogram_types.h

@@ -136,6 +136,7 @@ typedef struct {
     float exportScale;
     char exportDir[4096];
     char exportMessage[256];
+    float exportMessageTimer;  // seconds the export message stays on screen
 
     // Visible viewport (time + frequency) and in-progress pan state.
     Viewport view;