Prevent startup hangs when restoring filesystem roots

[Icatme]

Closes #118

What changed

• clear both mdview.folders and mdview.lastFolder before React mounts when either contains a filesystem root
• reject filesystem roots in the folder picker and exclude them from recursive folder watchers
• preserve unrelated persisted settings and add localized feedback for every supported language
• add regression coverage for both folder-state fallback directions and watcher filtering

Root cause

Version 1.6.1 restores persisted folders into a recursive Tauri filesystem watcher. If a Windows drive root such as V:\ is restored, the watcher traverses the entire drive and startup can become unresponsive. Clearing only one persisted folder key is insufficient because hydration falls back through the other key.

Validation

• bun test — 47 passed
• bun run build
• cargo check --manifest-path src-tauri/Cargo.toml
• built marka.md.exe in release mode and launched it against the affected WebView profile; the window reached input-idle and remained responsive