chore: add root ESLint config

[MurzNN]

Description

Adds a root-level ESLint flat config so lint checks run without manual setup. This unblocks CI and local workflows that expect ESLint to be configured out of the box.

Key Changes

Core (form-core)

• N/A

React Adapter (form-react)

• N/A

Docs / Web (apps/web)

• N/A

Other

• Add eslint.config.js with flat config for **/*.js and **/*.{ts,tsx} using @eslint/js and typescript-eslint recommended presets
• Set unused-variable rules to warn for both JS and TypeScript
• Add @eslint/js and typescript-eslint as root dev dependencies

Type of change

• feat (New feature)
• fix (Bug fix)
• docs (Documentation changes)
• refactor (Refactoring code without changing behavior)
• test (Adding or updating tests)
• chore (Maintenance, CI/CD, or Ops-related changes)

Related Issues

• Closes [BUG] Tests fail on the latest Node.js versions 25, 26 #173

Test plan

• Run pnpm lint from the repo root and confirm ESLint runs without extra config
• Verify CI lint step passes with the new config

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