Fix zoom glitch and add smart zoom for margin cropping

[Copilot]

Zoom in/out from fit-to-width mode used the stale zoomScale (default 1.0) as base instead of the actual PDFView scale factor, causing erratic jumps. Smart zoom added to maximize screen utilization.

Root cause

When fitToWidth = true, PDFKit's autoScales manages the scale factor independently (e.g. 0.7×). Pressing + computed 1.0 * 1.25 = 1.25 instead of 0.7 * 1.25 = 0.875, producing a jarring jump.

Changes

• Scale factor sync: LectorPDFView now observes PDFViewScaleChanged and writes the actual scaleFactor back to AppState.viewScaleFactor
• Zoom step fix: zoomIn/zoomOut use viewScaleFactor as base when transitioning from fit-to-width, zoomScale otherwise

  case .zoomIn:
      let base = fitToWidth ? viewScaleFactor : zoomScale
      fitToWidth = false
      zoomScale = min(base * 1.25, 10.0)

• Smart zoom (w key, :smartzoom): Toggles between fit-to-width and a tighter zoom that eliminates margin whitespace by comparing the page's cropBox to its mediaBox and scaling by the width ratio

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