Promote shared primitives into atlas-ui + default controls to compact density

[p-hoffmann]

Summary

Two related changes to the atlas-ui component library.

1. Promote shared primitives into atlas-ui (+ backfill stories)

Move the dependency-free generic components out of components/shared into the atlas-ui library and document them in Histoire:

• CollapsibleSection → ui/AtlasCollapsibleSection
• BuilderActionToolbar → ui/AtlasActionToolbar (themed divider colour so it adapts to dark mode)

Both gain index.ts exports and .story.vue/.story.md, and switch to direct sibling imports to avoid a barrel self-import cycle. Consumers updated: CohortGenerationSection and the five builder/editor pages.

Also:

• Add the missing AtlasFeedbackBody story.
• Fold the single-use LoadingSpinner into an inline AtlasProgressCircular in ConceptSearchDialog and delete the component + spec.

ErrorBoundary deliberately stays in shared/ — it depends on the locale store, router and app i18n keys, so it is app infrastructure rather than a reusable primitive (and cannot render in the Pinia-less Histoire setup).

2. Default all controls to compact density

The form controls already resolve to compact. Add density: tokens.density.default (compact) to the VBtn theme defaults so buttons match — making the densest option the default across all controls, while staying overridable per-instance. AtlasFab opts out with an explicit density="default" so it stays a full-size target.

Verification

• vue-tsc type-check: clean
• ESLint on changed files: clean
• AtlasCollapsibleSection spec (moved to tests/unit/components/ui/): 7/7 pass
• ConceptSearchDialog spec: 34/34 pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.69%. Comparing base (c98b210) to head (3f8406a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #113   +/-   ##
========================================
  Coverage    92.69%   92.69%           
========================================
  Files          443      442    -1     
  Lines        94685    94668   -17     
  Branches     10648    10648           
========================================
- Hits         87764    87749   -15     
+ Misses        6921     6919    -2     

Flag	Coverage Δ
unit	92.69% <100.00%> (+<0.01%)	⬆️

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