fix: resolve remaining desktop queue issues

[BunsDev]

What

• finish the remaining error-legibility fixes from fix: increase error text size and contrast for legibility #13 without regressing terminal contrast
• replace macOS-only shortcut labels with generic cross-platform labels like Cmd/Ctrl
• make local desktop builds and secrets scanning behave correctly on Windows-friendly setups
• move full packaging to desktop:package while keeping pnpm build as a reliable verification build
• update docs to match the shipped shortcut language

Why

Two open PRs were still blocking the queue:

• fix: increase error text size and contrast for legibility #13 had valid text-size improvements, but its terminal color change reduced contrast
• Cross-platform shortcuts, onboarding, and build #9 had useful cross-platform work, but it was conflicting and its runtime platform formatting path was hydration-risky in this Next.js app

This PR lands the safe parts from both on top of current main.

Closes #4
Supersedes #13
Supersedes #9

Verification

• pnpm check
• pnpm test
• pnpm build