feat(deck): bind host game preview selection

[papi-ux]

Summary

• Binds the Deck preview shell selected demo host and selected read-only library game into one deterministic typed preview contract.
• Adds resolveLaunchPreviewBinding(...) so UI state derives selected host/game IDs, host detail, game card, typed intent, preview text, inert CTA, and copy-only action together.
• Updates QML focus/selection behavior so host/game focus changes refresh preview text locally while staying preview-only.

Scope / safety

• Preview-only UI binding; no backend launch.
• No Moonlight execution, host discovery, pairing, HostStore persistence, network calls, process spawning, or real game launch.
• Copy action remains local clipboard/inspection only.

Validation

• git diff --check origin/master
• bash scripts/check-public-surface.sh
• cmake --build build/deck-fallback-packaging -j$(nproc)
• ctest --test-dir build/deck-fallback-packaging --output-on-failure
• cmake --build build/deck-qt-packaging -j$(nproc)
• ctest --test-dir build/deck-qt-packaging --output-on-failure
• ./build/deck-qt-packaging/nova_deck_layout_test

Public surface scan

• local paths: 0
• LAN/private hosts: 0
• secret assignments/private keys: 0
• process execution APIs: 0
• network APIs: 0