[NO-ISSUE] fix(webkit): add border to avatar initials variant

[gab-az]

Summary

Adds a border to the initials variant of the avatar component. The border renders only when the initials fallback is shown — i.e. a label is set and no src image is present. It uses the --border-default token and inherits the root's shape (kind: circle | square), so it does not appear for the image or icon fallbacks.

Notes

• Single-line change in avatar.vue; no props, events, slots, or exports added.
• No new dependencies. No breaking changes.

[robsongajunior]

Success regression tests:
https://github.com/aziontech/webkit/actions/runs/29524742617/job/87711048405?pr=750

[TEST] Summary of all failing tests
[TEST] FAIL apps/storybook/src/stories/components/content/avatar/Avatar.stories.js
[TEST]   ● Components/Content/Avatar › Default › smoke-test
[TEST] 
[TEST]     Expected image to match or be a close match to snapshot but was 1.26953125% different from snapshot (52 differing pixels).
[TEST]     See diff for details: /home/runner/work/webkit/webkit/apps/storybook/.storybook/test-visual/__diff_output__/components-content-avatar--default-diff.png
[TEST] 
[TEST]       69 |
[TEST]       70 |     const image = await page.locator('#storybook-root').screenshot({ animations: 'disabled' })
[TEST]     > 71 |     expect(image).toMatchImageSnapshot({
[TEST]          |                   ^
[TEST]       72 |       customSnapshotsDir: SNAPSHOT_DIR,
[TEST]       73 |       customSnapshotIdentifier: context.id,
[TEST]       74 |       customDiffDir: DIFF_DIR,
[TEST] 
[TEST]       at toMatchImageSnapshot (apps/storybook/.storybook/test-runner.js:71:19)
[TEST] 
[TEST]   ● Components/Content/Avatar › Types › smoke-test
[TEST] 
[TEST]     Expected image to match or be a close match to snapshot but was 1.184640522875817% different from snapshot (145 differing pixels).
[TEST]     See diff for details: /home/runner/work/webkit/webkit/apps/storybook/.storybook/test-visual/__diff_output__/components-content-avatar--types-diff.png
[TEST] 
[TEST]       69 |
[TEST]       70 |     const image = await page.locator('#storybook-root').screenshot({ animations: 'disabled' })
[TEST]     > 71 |     expect(image).toMatchImageSnapshot({
[TEST]          |                   ^
[TEST]       72 |       customSnapshotsDir: SNAPSHOT_DIR,
[TEST]       73 |       customSnapshotIdentifier: context.id,
[TEST]       74 |       customDiffDir: DIFF_DIR,
[TEST] 
[TEST]       at toMatchImageSnapshot (apps/storybook/.storybook/test-runner.js:71:19)
[TEST] 
[TEST]   ● Components/Content/Avatar › Sizes › smoke-test
[TEST] 
[TEST]     Expected image to match or be a close match to snapshot but was 1.0714285714285714% different from snapshot (144 differing pixels).
[TEST]     See diff for details: /home/runner/work/webkit/webkit/apps/storybook/.storybook/test-visual/__diff_output__/components-content-avatar--sizes-diff.png
[TEST] 
[TEST]       69 |
[TEST]       70 |     const image = await page.locator('#storybook-root').screenshot({ animations: 'disabled' })
[TEST]     > 71 |     expect(image).toMatchImageSnapshot({
[TEST]          |                   ^
[TEST]       72 |       customSnapshotsDir: SNAPSHOT_DIR,
[TEST]       73 |       customSnapshotIdentifier: context.id,
[TEST]       74 |       customDiffDir: DIFF_DIR,
[TEST] 
[TEST]       at toMatchImageSnapshot (apps/storybook/.storybook/test-runner.js:71:19)
[TEST] 
[TEST] 
[TEST] Snapshot Summary
[TEST]  › 3 snapshots failed from 1 test suite. Inspect your code changes or re-run jest with `-u` to update them.