chore(frontend): flat ESLint config scopes custom rules to src only; decide tests-e2e lint scope #1668

Discovered while gating #1635 (ESLint 10 flat config, burndown run 2026-11-04).

Summary

After #1635, frontend/eslint.config.mjs applies the repo's custom rules (@typescript-eslint/no-explicit-any as warn, the ^_ unused-vars patterns) only to src/**/*.ts. Under the old .eslintrc.json they applied to any file passed in, so tests-e2e/*.spec.ts and playwright.config.ts now get the stricter recommended defaults (any is an error, _x is flagged) when linted explicitly.

Current behaviour

No current impact: neither npm run lint nor CI lints tests-e2e (see also #1645 for the unquoted src/**/*.ts glob).

Expected behaviour

Decide the lint scope: either lint tests-e2e and the Playwright config in CI with the same custom rules, or document that they are intentionally unlinted.

Proposed fix

Add a flat-config block for tests-e2e/**/*.ts and playwright.config.ts mirroring the src rules (or files: ['**/*.ts']), and extend the lint script to include tests-e2e once #1645 quotes the glob.

References

• chore(deps): Bump eslint from 8.57.1 to 10.13.0 in /frontend #1635 (gate record notes this), fix(frontend): npm run lint skips top-level src files (unquoted eslint glob) #1645. Severity: low.