feat(avatar): add RuiAvatar and RuiAvatarGroup components

[kelsos]

Summary

• Add RuiAvatar with image-first rendering, initials/icon/slot fallback, six size tokens (xs–2xl) plus arbitrary numeric sizes, three variants (circular/rounded/square), and context-color theming.
• Add RuiAvatarGroup that overlaps children with a configurable spacing token, slices to max with a +N surplus (overridable via total or the surplus slot), and provides size/variant to children via injection.
• Wire up the example app page (/avatars), Storybook stories, and Playwright e2e coverage.

Closes #144

Test plan

• pnpm run typecheck
• pnpm --filter @rotki/ui-library run test:run (28 avatar unit tests pass)
• pnpm run build:prod
• PLAYWRIGHT_CHROMIUM_PATH=/usr/bin/chromium pnpm test:e2e avatar.spec.ts (10/10 pass)
• Visual review in Storybook (pnpm run storybook) — Default, WithImage, Initials, WithIcon, BrokenImage, Sizes, Variants, Colors, NumericSize, WithStatusBadge
• Visual review of /avatars example page across sizes, variants, colors, and group cases

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.70629% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.87%. Comparing base (36e8be7) to head (962dd5a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...-library/src/components/avatars/RuiAvatarGroup.vue	86.27%	7 Missing ⚠️
.../ui-library/src/components/avatars/avatar-props.ts	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
+ Coverage   84.60%   84.87%   +0.26%     
==========================================
  Files         139      143       +4     
  Lines        5066     5209     +143     
  Branches     1507     1553      +46     
==========================================
+ Hits         4286     4421     +135     
- Misses        780      788       +8     

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