feat: add Prettier for code formatting (closes #27)

[Ai-chan-0411]

Summary

• Adds Prettier (v3) as a code formatter with a standard configuration
• Integrates eslint-config-prettier to disable ESLint rules that conflict with Prettier
• Adds format and format:check npm scripts for formatting and CI validation
• Formats all existing source files with Prettier for consistent code style

closes #27

Changes

File	Description
.prettierrc.json	Prettier configuration (semi, double quotes, trailing commas, 80 char width)
.prettierignore	Ignores build artifacts and lock files
package.json	Added prettier + eslint-config-prettier devDependencies and format scripts
eslint.config.js	Added eslint-config-prettier as last config entry to disable conflicting rules
*.ts, *.tsx, *.js, *.json, *.css, *.md	Auto-formatted by Prettier

Test plan

• npx prettier --check "**/*.{ts,tsx,js,jsx,json,css,md}" passes
• ESLint config still works with Prettier integration
• No functional code changes — formatting only

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[github-actions]

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