fix: align packaged app binary name

[mattenarle10]

Summary

• sets Tauri mainBinaryName to marka.md
• keeps the public product name, window title, app bundle name, and download naming aligned around marka.md

Closes #44 when merged.

Why

The existing macOS bundle already had:

• CFBundleName: marka.md
• CFBundleDisplayName: marka.md
• CFBundleExecutable: marka

That executable name can surface as the code-name-looking app name in places like Spotlight. This makes the release binary name explicit too.

Checks

• bun test
• bun run build
• git diff --check
• bun run tauri build --no-bundle

The Tauri no-bundle build completed and produced:

src-tauri/target/release/marka.md