Fix IIIF image viewer rendering

[ewlarson]

Summary

• allow IIIF image viewers to zoom out below native zoom 0 so tall CONTENTdm scans can fit the viewport
• clamp actual IIIF tile requests to native zooms and resize loaded tiles to their natural dimensions
• remove the opacity slider from IIIF info.json viewers while keeping fullscreen

Root Cause

The custom IIIF Leaflet path kept the map and tile layer at minZoom: 0, while the old GeoBlacklight leaflet-iiif plugin synthesized lower overview levels for large images. Tall scans such as pstems_0052767067_altoona_04_lk could not fit the viewer and non-1024 overview/edge tiles were being laid out at the wrong CSS size.

Tests

• /Users/ewlarson/.cache/codex-runtimes/codex-primary-runtime/dependencies/node/bin/node node_modules/vitest/vitest.mjs run src/__tests__/geoblacklight/leaflet_viewer_controller.test.js