feat(Viewer): add zoom and pan to image viewer

[vladopol]

Summary

Adds zoom and pan support to the image viewer in Talk Desktop, addressing the long-standing macOS limitation where pinch-to-zoom had no effect and Cmd+/- zoomed the entire application UI instead of the image.

Installs panzoom (v9.4.4, 6 kB) — same library already used in spreed for screen share zoom (spreed#14028).

Changes in ViewerHandlerImages.vue:

• Scroll wheel / trackpad pinch → zoom in/out (1× – 8×), centered on cursor
• Drag to pan when zoomed in
• Double-click to zoom in 3× at cursor position; double-click again to reset to 1×
• Auto-centers image when zoom returns to 1× (smooth animation)
• Panning blocked at minimum zoom to prevent accidental image shifts
• Cursor feedback: zoom-in at 1×, grab / grabbing when zoomed

Test plan

• Open an image in the viewer
• Scroll wheel / trackpad pinch zooms image centered on cursor (not top-left corner)
• Drag image while zoomed in — panning works, cursor shows grab/grabbing
• Zoom back out to 1× — image smoothly returns to center
• Double-click zooms in 3× at click point; double-click again resets
• Open a second image — zoom/pan state resets
• Verify no regression on video viewer (ViewerHandlerVideos.vue unchanged)

Closes #1535

[nickvergessen]

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