[FEAT] Add cross-platform support (Windows) for configuration and storage paths

[daniil-verba]

[FEAT] Add cross-platform support (Windows) for configuration and storage paths

📝 Description

Currently, the project's storage and configuration logic (e.g., in storage/*.rs) is hardcoded to follow Linux filesystem standards (like ~/.config/undertow). As a result, the protocol fails to start or save data correctly on Windows environments.

This issue aims to refactor the path resolution logic to be fully cross-platform, with immediate focus on adding robust Windows support.

💡 Motivation

• Expand User Base: A P2P protocol should be usable by anyone, regardless of their OS. Currently, Windows users cannot use undertow-protocol.
• Developer Experience: Many contributors and testers primarily use Windows. Making the project build and run out-of-the-box on Windows will lower the barrier to entry for new contributors.
• Prevent Panics: Prevents runtime errors/panics when the application tries to access non-existent Linux paths on a Windows machine.

⚙️ Technical Details

Instead of manually concatenating strings or relying on std::env::var("HOME"), we should use an established, cross-platform crate to resolve standard directories.

Implementation Steps:

1. Add Dependency: Add the directories (or dirs) crate to Cargo.toml.
2. Refactor storage/*.rs: Replace hardcoded Linux paths with OS-aware path resolution.
   Example:

   use directories::ProjectDirs;
   
   // This resolves to:
   // Linux: ~/.config/undertow-protocol
   // Windows: C:\Users\Username\AppData\Roaming\undertow-protocol
   // macOS: ~/Library/Application Support/undertow-protocol
   let proj_dirs = ProjectDirs::from("com", "undertow", "protocol").unwrap();
   let config_dir = proj_dirs.config_dir();

3. Testing: Verify that the application successfully creates the directory and reads/writes config files on a Windows machine (or via cross-compilation check: cargo check --target x86_64-pc-windows-msvc).

🔄 Alternatives Considered

• Manual #[cfg(target_os = "...")] macros: Writing custom if/else blocks for every OS. Rejected: Reinvents the wheel, prone to edge-case bugs, and harder to maintain if we add macOS support later.
• Using the Current Working Directory (./config.json): Rejected: This is bad practice for background services or libraries, as it clutters the user's execution directory and depends on where the binary was launched from.

📎 Additional Context

• Affected Files: Primarily storage/*.rs (and any other modules assuming Linux paths).
• Recommended Crate: directories v5.0+
• Future Scope: While this issue focuses on Windows, using ProjectDirs will automatically lay the groundwork for proper macOS support in the future.

✅ Acceptance Criteria

• directories (or dirs) crate is added to Cargo.toml.
• All hardcoded Linux paths in storage/ are replaced with OS-agnostic resolution.
• The project successfully compiles for Windows (cargo check --target x86_64-pc-windows-msvc).
• Manual testing confirms config files are saved to %APPDATA% (or equivalent) on Windows.