Fix ChatGPT Tab suggestion acceptance

[bartekplus]

Summary

• add a document_start main-world Tab bridge for rich editors so pages like ChatGPT cannot steal focus before FluentTyper accepts a visible suggestion
• route early bridge messages through the content-script runtime, return real accept results, and retry contenteditable accepts when a host cancels beforeinput without mutating the DOM
• default unset Tab/Enter acceptance settings to true and scope the early bridge away from plain inputs so existing input/textarea Tab acceptance keeps the normal path

Test Plan

• bun run lint
• bun run format:check
• bun run test
• bun run check:e2e:coverage
• bun run test:e2e
• bun run test:e2e --platform=firefox
• bun run test:e2e:full
• bun run test:e2e:full --platform=firefox