Migrate to TypeScript 7 and Oxlint

[bartekplus]

Summary

• replace TypeScript 6 with TypeScript 7.0.2
• replace ESLint and typescript-eslint with type-aware Oxlint
• preserve the existing architecture import boundaries and lint policy
• fix all 83 unnecessary type assertions instead of disabling typescript/no-unnecessary-type-assertion
• update CI and contributor documentation for the new toolchain

Why

The previous lint stack depended on TypeScript 6-compatible typescript-eslint packages. Moving type checking and linting together to TypeScript 7 and Oxlint removes that compatibility split and leaves a single TypeScript compiler version in the dependency tree.

Impact

bun run check now runs type-aware Oxlint, Prettier, and TypeScript 7. ESLint, typescript-eslint, their support packages, and eslint.config.js are removed. Runtime behavior remains unchanged; the source edits remove assertions that the type system proved unnecessary.

Validation

• bun run check
• bun run test — 1,561 passing
• bun run check:e2e:coverage
• bun run build
• bun run build --platform=firefox
• bun run test:e2e --platform=chrome — 26 passing
• bun run test:e2e --platform=firefox — 26 passing
• git diff --check