fix(windows): embed icon in executable

[rteoo]

Summary

• Embed the existing disktree mark in Windows executables through the Windows SDK resource compiler.
• Add a Windows CI check that fails if the release executable has no embedded icon.

Closes #31

Testing

• Compiled the icon resource with Windows SDK rc.exe on Windows and verified the ICO contains 16, 32, 48, and 256 px sizes.
• Validated CI YAML and ran git diff --check.
• The identical corrected build.rs passed rustfmt and Clippy in the fork's CI run; all six platform checks passed, including the Windows release executable icon assertion.
• Upstream has not reported checks on this PR yet.

Risks

• Windows builds now require the Windows SDK resource compiler, which is included with the MSVC SDK already needed by the app.