chore: update packages and migrate to ESLint v9 flat config

Package updates (non-breaking):
- Client: Updated @eslint/js, @stylistic/eslint-plugin, @tailwindcss/typography,
  chart.js, eslint, eslint-plugin-vue, firebase, vue, vue-router
- Functions: Updated firebase-functions, typescript

ESLint v9 migration:
- Upgraded ESLint from v8 to v9 in functions
- Upgraded @typescript-eslint packages to v8
- Migrated from .eslintrc.js to flat config (eslint.config.js)
- Removed eslint-config-google (not compatible with ESLint v9)
- Updated lint script to remove deprecated --ext flag

All builds verified working after updates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lhermann