docs: streamline README and extract CONTRIBUTING.md (#51)

- Reduce README from 491 to 285 lines (42% less)
- Remove duplicated sections (Usage Tips, Support & Contact)
- Move dev setup, code standards, PR process to CONTRIBUTING.md
- Replace What's New with link to CHANGELOG_USER.md
- Trim Features section (remove dev jargon, condense parental controls)
- Replace MPV keyboard table with link to MPV docs
- Remove static tests-passing badge
- Move legal disclaimer to prominent position
- Clean up marketing language

Author: mewset

CONTRIBUTING.md

@@ -0,0 +1,73 @@
+# Contributing to Better IPTV
+
+## Report Bugs
+
+[Create an issue](https://github.com/mewset/better-iptv/issues/new) with:
+- Detailed description
+- Steps to reproduce
+- OS and app version
+- Screenshots if applicable
+- Log file (see [Troubleshooting](README.md#%EF%B8%8F-troubleshooting))
+
+## Suggest Features
+
+[Open a feature request](https://github.com/mewset/better-iptv/issues/new) describing:
+- What you want
+- Why it's useful
+- How it should work
+
+## Development Setup
+
+```bash
+# Fork & clone
+git clone https://github.com/YOUR-USERNAME/better-iptv.git
+cd better-iptv
+
+# Install dependencies
+npm install
+
+# Run dev server
+npm run tauri dev
+
+# Run tests
+npm run test          # Frontend tests
+cd src-tauri && cargo test  # Rust tests
+```
+
+### Build from Source
+
+```bash
+npm run tauri build
+# Output: src-tauri/target/release/bundle/
+```
+
+## Code Standards
+
+- **TypeScript**: Follow ESLint config (`npm run lint`)
+- **Rust**: Use `rustfmt` and `clippy`
+  ```bash
+  cargo fmt
+  cargo clippy
+  ```
+- **Commits**: Use [Conventional Commits](https://www.conventionalcommits.org/)
+  ```
+  feat: add category quick-access bar
+  fix: resolve EPG timezone bug
+  docs: update README installation steps
+  ```
+
+## Pull Request Process
+
+1. Create feature branch: `git checkout -b feature/my-feature`
+2. Make changes with tests
+3. Run linters: `npm run lint && cargo clippy`
+4. Commit: `git commit -m "feat: description"`
+5. Push: `git push origin feature/my-feature`
+6. Open PR on GitHub with detailed description
+
+## Community Guidelines
+
+- Be respectful and inclusive
+- Provide constructive feedback
+- Help other users in issues/discussions
+- Document your changes clearly