Focus the existing window when opening Markdown files

[daniel-grabowski]

Summary

• Route file-open requests from secondary app launches through the already running main window.
• Show, unminimize, and focus the window after receiving a Markdown file path so the opened document is visible immediately.
• Keep the Windows target check warning-free by marking the macOS-only clipboard scaling parameter as intentionally unused on other platforms.

Issue

fixes #191

Testing

• cargo xwin check --target x86_64-pc-windows-msvc
• git diff --check

Notes

• This PR is intentionally limited to the Tauri window/file-open path. It does not include local release artifacts or process journal files.