Fix platform-specific Rollup binary in package.json dependencies

[Zena-park]

Context

Flagged in PR #35 code review (comment).

Problem

crates/desktop-app/ui/package.json includes an OS/arch-specific Rollup binary (@rollup/rollup-darwin-arm64) as a direct dependency. This will break installs on non-macOS ARM64 platforms (e.g., CI on Linux x64).

Proposed Fix

1. Remove the direct dependency and rely on Rollup/Vite to manage platform-specific binaries automatically
2. Or move it to optionalDependencies with appropriate platform handling
3. Verify CI builds still work on all target platforms after the change