docs: Add a "Troubleshooting / Common Setup Issues" section to the README

Summary New contributors keep hitting the same setup snags. Collect them into one Troubleshooting section in the README so people can self-serve.

Where

• README.md — add a ## 🩹 Troubleshooting section near the end of the Development section.

Steps

1. Fork and branch: git checkout -b docs/readme-troubleshooting.
2. Add a ## 🩹 Troubleshooting section covering:
   • Tauri prerequisites per OS — link https://tauri.app/start/prerequisites/ and list common Linux packages (webkit2gtk, librsvg).
   • Wrong Node/Yarn version — Node 20+ and Yarn Classic (v1); how to check (node -v, yarn -v).
   • macOS "app is damaged / unidentified developer" — the xattr -dr com.apple.quarantine workaround (cross-link the Download note).
   • Python runtime not found — short note + link to #238.
3. Run yarn lint.
4. Commit docs: add troubleshooting section to README and open a PR.

Acceptance criteria

• README has a Troubleshooting section with at least the four items above.
• yarn lint passes.

Good first issue — comment here if you'd like to take it.