Handle malformed Unicode in generated avatars

[ThumulaPerera]

Purpose

generateAvatarDataUri didn't handle some valid inputs correctly:

• Emoji cut in half. Text-avatar initials were cut with String.prototype.slice, which counts UTF-16 code units. When the cut fell inside an emoji or another character outside the Basic Multilingual Plane, it left a lone surrogate. For example, content: 'B😈' with lead to a lone surrogate causing rendering issue, and content: '😈B' lost its second letter.
• Invalid SVG. Initials were inserted into the SVG without XML escaping. Content containing & or < produced invalid SVG, which renders as a broken image.

Approach

All changes are in packages/browser/src/utils/generateAvatarDataUri.ts:

• Initials are cut by code point (Array.from), so a character is never split.
• Initials are XML-escaped before they're inserted into the <text> element.
• A new toSvgDataUri helper replaces any remaining lone surrogate with U+FFFD before encoding. All three generators (gradient, anonymous animal, anonymous entity) use it, so none of them can throw on caller-supplied content or bg.
• The surrogate check uses Array.from rather than a /\p{Surrogate}/u regex, so the package doesn't need any newer JavaScript features than it does today.

Related Issues

• N/A

Related PRs

• N/A

Checklist

• Followed the contribution guidelines.
• Manual test round performed and verified.
• Documentation provided. (Add links if there are any)
• Tests provided. (Add links if there are any)
  • Unit Tests
  • Integration Tests
• Breaking changes. (Fill if applicable)
  • Breaking changes section filled.
  • breaking change label added.
• Cross-SDK parity. Exactly one of parity/prs-raised or parity/prs-not-needed added.
  • If parity/prs-raised, the port links are posted as a reply on the parity check's comment.

Security checks

• Followed secure coding standards.
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets.

[github-actions]

🔀 Cross-SDK feature parity, no ports needed

This pull request is labelled parity/prs-not-needed: nothing here has to ship in the other ThunderID SDKs. If that changes, swap the label for parity/prs-raised.

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