chore: enforce repository guidance

[ArionMiles]

Description

Consolidate repository guidance, make key backend and frontend rules enforceable, and remove obsolete screenshot maintenance paths.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Build/CI configuration change

Related Issue

None.

Changes Made

• Replace tool-specific instruction files with focused root, frontend, and screenshot guidance.
• Add backend policy checks and ESLint enforcement for TypeScript, React Hooks, React Refresh, accessibility, and frontend component policies.
• Fix existing accessibility and hook findings, upgrade Playwright, and remove the obsolete mocked README screenshot workflow.

Testing

• Unit tests pass (task test)
• Linter passes (task lint and task lint:be:prod)
• Browser tests pass (task test:fe:e2e, 15 tests)
• Dependency audit passes (task audit)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

Not applicable.

Additional Notes

Fast Refresh remains strict for new modules. An explicit allowlist covers established modules that intentionally combine component and support exports.