feat: live cursor readout (time / frequency / STFT level)

Floating tag follows the pointer over the spectrogram showing the time,
frequency, and STFT magnitude (dB) under the cursor — the standard
spectrum-analyzer probe. Suppressed while selecting/panning (which have
their own readout) and when a modal is open.

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

src/spectrogram.c

@@ -964,6 +964,7 @@ int main(int argc, char* argv[])
             DrawSelectionDrag(viewBounds);
             DrawPlayhead(viewBounds);
             DrawLabels(viewBounds);
+            if (!UiModalOpen()) DrawCursorReadout(viewBounds);
             float maxFreq = (float)app.signal.sampleRate / 2.0f;
             float freqMin = app.view.freqStart * maxFreq;
             float freqMax = app.view.freqEnd * maxFreq;