chore: centralize pre-commit hooks (husky + lint-staged)

Summary

• Centralize husky + lint-staged at root package.json (was only in frontend)
• lint-staged runs eslint + prettier per-component on staged files only
• Gracefully skips if node_modules not installed, with informative warning
• Remove frontend-local husky dep, lint-staged dep, prepare script

Test plan

• Commit in worktree without node_modules → warning displayed, commit succeeds
• Commit with node_modules installed → eslint + prettier run on staged files
• CI lint stage passes