feat(checkbox-group): add RuiCheckboxGroup component

[kelsos]

Summary

• Adds RuiCheckboxGroup — a multi-select grouping primitive with v-model array of selected values, mirroring RuiRadioGroup's public API (label, hint, errorMessages, successMessages, hideDetails, disabled, color, size, required, inline).
• RuiCheckbox gains an optional generic value prop and joins a group via provide/inject when nested under RuiCheckboxGroup. Outside a group it stays a plain boolean checkbox (backward compatible).
• Includes Storybook stories, unit tests, an example app section, and e2e coverage.

Closes #50.

Design notes

• Provide/inject (rather than slot-VNode iteration like RuiRadioGroup) keeps RuiCheckbox's public modelValue typed as boolean, so existing internal consumers (RuiDataTable, RuiTableHead) are unaffected.
• The injected context uses MaybeRefOrGetter<T> + toValue() for group-level disabled/color/size propagation.
• Children inside a group auto-suppress their per-row RuiFormTextDetail so the group's own hint/error renders cleanly.

Test plan

• pnpm typecheck (lib + example) — clean
• pnpm lint — clean (0 errors)
• pnpm --filter @rotki/ui-library test:run — 1080/1080 unit tests pass
• pnpm --filter example test:e2e:dev forms/checkbox.spec.ts — 5/5 pass
• pnpm --filter @rotki/ui-library build:prod — builds cleanly, web-types regenerated
• Reviewer: spot-check Storybook entries (Components/Forms/Checkbox/CheckboxGroup)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.78%. Comparing base (41038be) to head (d7928d1).

Files with missing lines	Patch %	Lines
...forms/checkbox/checkbox-group/RuiCheckboxGroup.vue	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #524      +/-   ##
==========================================
+ Coverage   84.73%   84.78%   +0.05%     
==========================================
  Files         143      145       +2     
  Lines        5227     5259      +32     
  Branches     1556     1570      +14     
==========================================
+ Hits         4429     4459      +30     
- Misses        798      800       +2     

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