chore(frontend): add missing ESLint config and wire lint into CI (salvaged from #1331) #1350

Salvaged from #1331 (being closed as superseded by #1345): the frontend declares a lint script (eslint src/**/*.ts) and carries eslint + @typescript-eslint deps, but NO ESLint config exists anywhere in the repo (no .eslintrc*, no eslintConfig in package.json), so npm run lint is non-functional. #1331 contained a 10-line frontend/.eslintrc.json wiring @typescript-eslint/parser as the root config - that file is the useful residual.

Scope: (1) add the ESLint config (compatible with the typescript-eslint v8 flat/legacy config expectations after #1345 merges - v8 prefers flat config eslint.config.js; decide format accordingly rather than copying #1331's legacy .eslintrc.json verbatim); (2) make npm run lint pass on src/ (fix or explicitly configure rules for existing findings - no blanket disables); (3) wire it into the frontend CI job so it actually gates.