fix: polish cross-platform commit shortcut

[danielss-dev]

Summary

• render the Windows/Linux commit shortcut as compact Ctrl and return keycaps
• keep macOS on its native command/return glyph treatment
• preserve Linux as a distinct platform while formatting its shortcuts with Ctrl

Test plan

• pnpm --filter ./ui exec tsc --noEmit
• pnpm --filter ./ui exec vitest run src/lib/keys.test.ts
• git diff --check
• visually verified the commit chord in the running Windows app