fix: add error boundary, safe base64 encoding, and proper fetch error handling

[avinxshKD]

this fixes #342, error boundary at root so crashes dont white screen, switched btoa to TextEncoder so emoji labels dont break autosave, fixed fetch rejects that were passing a promise instead of a string so catch blocks were getting

[avinxshKD]

@pradeeban fixing this was a bit weird to debug because btoa and fetch reject paths were failing in non-obvious ways, but should be stable now.

btoa was the main footgun here (emoji labels could silently break autosave), fetch rejects were passing unresolved bodies so catches got useless errors, and root boundary now prevents full white-screen crashes... happy to scope the boundary lower if you prefer lmk thanks

[avinxshKD]

Also i tried to keep it as minimal i can, and stay within scope, still learning