[Feature] macOS: support Intel Macs (universal binary)

[thangnm93]

Problem Statement

macOS releases were Apple Silicon only (macOS-arm64). Intel Macs could not run the app at all, and the in-app updater deliberately returned no asset on Intel (browser fallback). Intel users were locked out of the macOS build entirely.

Proposed Solution

Ship a single universal (arm64 + x86_64) artifact built on the arm64 runner:

• packages/yourssh_rdp/build.sh builds the Rust dylib for both darwin targets and lipos them into one universal libyourssh_rdp.dylib
• Flutter already builds both slices by default; the release workflow renames macOS artifacts to -macOS-universal and adds a lipo -archs assertion over the app binary and the dylib, so an arm64-only artifact named universal can never ship
• UpdateService: arm64 prefers the universal DMG and falls back to the arm64-only name of older releases; Intel matches only the universal DMG and falls back to the browser against pre-universal releases

Alternatives Considered

• Separate Intel build on GitHub's Intel runners (macos-13 / macos-15-intel) — previously dropped due to chronic runner unavailability; also doubles CI time and produces a second artifact users must pick correctly
• Staying arm64-only — keeps excluding the Intel install base for a modest download-size saving

Additional Context

Implemented in #61. Trade-off: the universal DMG is larger than the arm64-only one (two slices), offset by the symbols split and font cut from #63. First real verification happens on the next release run via the new lipo assertion step.