Prototype: pointer-centred image zoom and drag pan

[GeoffRiley]

Parent checkpoint: #2. Depends on #6.

Implement the image interaction in the right pane, using the two sample pages during development.

Acceptance

• Wheel zoom keeps the image point under the pointer approximately fixed.
• Drag panning works without accidental text edits or jumps.
• Fit image and 100% reset work after resize and reopening.
• Zoom level is visible and sensible limits prevent disappearing or huge scales.
• A short manual test records behavior on a large real image as well as samples.

If the controls feel awkward, revise before adding folder workflow.