Add octave offset dropdown, fix waveform sidebar, embed audio in plugin state

- Add octave offset combo (-3 to +3) in title bar for MIDI input transposition
- Fix waveform sidebar base note from C3 to C4
- Fix midiNoteToSliceIndex to compare pitch classes instead of raw MIDI notes
- Add colored slice indicator dots on waveform sidebar keys
- Embed audio data, file path, sample rate, and slices in plugin state for
  DAW project save/restore (falls back to file path if embedded data missing)

Source/SliceManager.h

@@ -12,7 +12,7 @@ public:
     {
         int startSample;
         int endSample;
-        int midiNote;   // halftone offset from C3 (0 = C3, 1 = C#3, etc.)
+        int midiNote;   // pitch class (0 = C, 1 = C#, ..., 11 = B)
     };
 
     SliceManager();