fix(windows): show and activate custom style packs from tray menu

[H-Chris233]

Summary

• Build the Windows tray style menu from every enabled StylePack instead of four hard-coded polish modes.
• Use stable, collision-safe IDs derived from StylePack.id and mark the active pack by exact ID.
• Activate the selected pack through �ctivate_style_pack_by_id, while safely ignoring malformed, stale, or disabled menu IDs.
• Add focused tests for built-in/imported packs, duplicate base modes, disabled packs, and invalid IDs.

Validation

• cargo test --locked tray_style_menu — passed (4 tests).

pm test — passed (build plus 43 frontend/contract tests).

• Full cargo test --locked is currently blocked by the existing Windows symlink test when the environment lacks symlink creation permission; the unrelated proxy/cooldown failures pass when rerun individually.

Closes #808