fix(web): render oversized terminal graphemes without crashing - #7809

💭 This is a plain utility function for codepoint conversion, not an Effect service, so standard conventions don't apply here. I want to check the test file's import pattern against other tests in the repo to confirm it's consistent, and make sure I'm not adding unnecessary tests just to validate a mechanical refactor.

1. git_tools — from "vite-plus/test"

2. browse_code — apps/web/src/terminal/ghostty/core.ts

💭 This is just a pure helper function with no Effect service dependencies involved, so it's clean.