Adds compareStudies() to viewer.js:
- Switches to 2-panel layout
- Loads most recent study (studies[0]) in left panel
- Loads prior study (studies[1]) in right panel
- Auto-matches same orientation series (AX > SAG > COR)
- Enables sync scroll automatically
- Jumps both panels to middle slice
Adds 'Compare' button to viewer toolbar (viewer/main.go) next to
1/2/3 Panel and 3D buttons.
Sync scroll already worked across studies via slice_location matching —
this just makes the workflow one click.
f2e352ebcf