docs: improve README structure and readability (#3515)

* docs: Improve README.md with enhanced structure and content

- Add Quick Start and System Requirements sections
- Add Privacy badge to badge section
- Reorganize Features section by category (AI Model Support, User Experience, Content & Formatting, etc.)
- Improve Contributing section with detailed contribution guidelines
- Improve Development section with setup instructions, build commands, project structure, and troubleshooting

* fix: correct project structure, tool names and troubleshooting in README

- Fix project structure to match actual directory layout (src/main, src/renderer, src/preload, src/shared)
- Correct linter name from ESLint to Biome
- Correct test runner to Vitest
- Fix troubleshooting tip (remove non-existent npm run clean)

* fix: update package manager from npm to pnpm in README

The project has migrated to pnpm (packageManager: pnpm@10.15.1).
Updated all npm references to pnpm including:
- Prerequisites section
- Quick Setup commands
- Build Commands table
- Development Tips
- Troubleshooting section