Refactor storage & downloader

[arse09]

This PR is a refactor of storage handling and separating filesystem logic from downloader logic.

Summary

• Moved launcher back-end storage logic into src/storage.rs.
• Settings are now stored in settings.json.
• Folder structure:
  - accounts.json: Accounts data like username and UUID. Future pr.
  - installations.json: UI data of each installation for quick loading. PR: feat: Implement installations #62
  - installations/: Each installation's directory. PR: feat: Implement installations #62
  - settings.json: Launcher settings.
  - Probably servers.json but idk: Issue: Implement Servers page in launcher #45

Test plan

• Remove .pomc folder from your projects directory.
  - Linux: $XDG_DATA_HOME/.pomc (usually ~/.local/share/.pomc)
  - Windows: %APPDATA%\.pomc (usually C:\Users\<User>\AppData\Roaming\.pomc)
  - macOS: $HOME/Library/Application Support/.pomc
• Open your launcher and see the folder structure appear.
• To test settings saving:
• Go to settings.
• Change them.
• Close and open the launcher.

[Purdze]

data_dir() called repeatedly, every path function calls data_dir() which calls ProjectDirs::from() each time. Should cache it with LazyLock or OnceLock

Settings loaded from disk on every toggle, update_settings calls load() then save() on each click. Should keep settings in memory and only write on change

accounts.json is fine for non-sensitive data (username, UUID) but auth tokens should stay in the OS keyring where they're encrypted. Don't migrate tokens to a plaintext JSON file