fix: handle Unicode characters in auth emulator usernames

[keith-oak]

Summary

• Fixed issue where usernames containing Unicode characters (emojis, non-Latin text) would cause btoa() to throw an error
• Applied UTF-8 encoding before base64 encoding to properly handle Unicode characters

Changes

• Updated saveCookie() function in src/public/auth.html to use btoa(unescape(encodeURIComponent(data))) instead of btoa(data)
• This ensures Unicode characters are properly encoded to UTF-8 before base64 encoding

Test plan

• [x] Tested with various Unicode inputs including emojis (😊), Chinese characters (用户), and Arabic text (مستخدم)
• [x] Verified ASCII characters still work correctly
• [x] All existing unit tests pass

Fixes #269

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