test(desktop): make slash caret movement portable

[xxhZs]

Summary

• use the platform-native line-start shortcut in the slash-command menu E2E
• keep Home on Windows/Linux and use Meta+ArrowLeft on macOS
• make the caret-in-query assertion deterministic instead of depending on menu update timing

Root cause

The product command filter already rejects a slash when non-whitespace text remains after the caret. The flaky test attempted to place the caret after / with Home followed by ArrowRight. In macOS contentEditable, Home scrolls the document rather than moving to line start, so the caret remained at the end of /side; the assertion only passed when unrelated menu timing temporarily hid the command group.

Verification

• baseline focused repetition: 15/20 passed
• fixed focused repetition: 20/20 passed
• complete slash-command-menu.spec.ts: 4/4 passed
• Desktop typecheck
• Biome and git diff --check

Closes#2948

[Astro-Han]

LGTM.