Add double-click focus mode for image layers

[cornhundred]

Summary

• add an observable for the currently focused image layer
• handle double-clicking image layer buttons to focus or clear focus
• update UI controls to disable non-focused image buttons and sliders and sync layer visibility

Testing

• npm run test:js

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https://chatgpt.com/codex/tasks/task_b_68f0f8a460f8832a89eb25bd95627ebf

[cornhundred]

Add double-click focus mode for image layers

Double-clicking an image-layer button now isolates that layer: it becomes
the only visible image layer and the only interactive button/slider.
Double-clicking the focused layer again exits focus mode and restores all
layers.

Changes

• New focused_image_layer observable in the obs store
• text_buttons.js: dblclick handler on image-layer buttons that toggles
  focus; single-click callback no-ops while focus is active
• ui_containers.js: sync_image_layer_controls() centralizes button/slider/
  visibility state for both normal and focused modes; switches to
  toggle_visibility_single_image_layer for per-layer control
• switch_dataset.js: clear focus mode when switching datasets, so a focused
  layer name from the previous dataset can't disable all image buttons

Notes

• Merged latest main (image-layer refactor + dataset switching). The only
  conflict was an additive one in obs_store.js; resolved by keeping both
  sets of new observables.
• Rebuilt the distributed docs/assets/js/celldega.js bundle.
• Verified: ESLint clean, jest passes, node build.js succeeds.

Known follow-up (non-blocking)

• A dblclick fires two click events first, so entering focus mode runs the
  single-click image callback twice (state nets out, but triggers redundant
  deck refreshes). Could be smoothed with a click-suppress timer.
• No visual hint that double-click focuses a layer; a tooltip/title on the
  image buttons would aid discoverability.