feat(spectral_editor): Add cursor-centered zoom and pan with mouse wheel

Implemented zoom and pan system for the spectral editor:

Core Features:
- Viewport offset system (viewportOffsetX, viewportOffsetY) for panning
- Three wheel interaction modes:
  * Ctrl/Cmd + wheel: Cursor-centered zoom (both axes)
  * Shift + wheel: Horizontal pan
  * Normal wheel: Vertical pan
- Zoom range: 0.5-20.0x horizontal, 0.1-5.0x vertical
- Zoom factor: 0.9/1.1 per wheel notch (10% change)

Technical Implementation:
- Calculate data position under cursor before zoom
- Apply zoom to pixelsPerFrame and pixelsPerBin
- Adjust viewport offsets to keep cursor position stable
- Clamp offsets to valid ranges (0 to max content size)
- Updated all coordinate conversion functions (screenToSpectrogram, spectrogramToScreen)
- Updated playhead rendering with visibility check
- Reset viewport offsets on file load

Algorithm (cursor-centered zoom):
1. Calculate frame and frequency under cursor: pos = (screen + offset) / scale
2. Apply zoom: scale *= zoomFactor
3. Adjust offset: offset = pos * scale - screen
4. Clamp offset to [0, maxOffset]

This matches the zoom behavior of the timeline editor, adapted for 2D spectrogram display.

handoff(Claude): Spectral editor zoom implementation complete

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