chore: replace ESLint with oxlint

[ElbertePlinio]

Full 1:1 replacement — the only true migration in the workspace. All current rules (complexity 15, max-depth 4, max-lines-per-function 100 with test-file override) are verified supported by oxlint. Effort: S.

Agent plan

Goal: oxlint is the primary lint gate (edit loop + CI), per workspace AGENTS.md "Frontend tooling". Audit source: oxlint 1.80.0 run on this tree, 2026-08-25.

Checklist:

• Add .oxlintrc.json porting the 3 rules + *.test.ts override
• Decide on default correctness+unicorn categories (15 baseline findings: 8 unicorn/no-thenable in a test file, 4 no-unused-vars, 3 misc)
• Remove eslint, typescript-eslint, eslint.config.ts and their deps
• lint script runs oxlint; CI "Lint" step uses it
• Baseline is clean (fix findings or justify per-rule disables in .oxlintrc.json)
• PR/review

Validation:

• bunx oxlint (or npx oxlint) exits 0 on the tree
• CI green

Current status: Planned
Next action: open PR