feat(deck): use read-only library snapshot in Deck shell

[papi-ux]

Summary

• Adds a Deck-side read-only Polaris library snapshot boundary for host/game data.
• Populates the Deck shell with real-shaped host/game/detail data instead of pure fake/sample UI state.
• Refines controller focus wrapping, selected host/game affordances, empty/read-only copy, and the copy-preview CTA.

Scope / non-goals

• This does not add backend launch execution.
• This does not add streaming, host discovery, pairing, or HostStore persistence.
• This does not add Moonlight support, wiring, invocation, or dependency.
• Preview URIs remain display/copy-only artifacts.

Validation

• git diff --check origin/master...HEAD -- clients/deck
• Rebuilt focused fallback Deck test target and ran ctest: nova_deck_controller_library_smoke passed.
• Rebuilt focused Qt Deck target and ran ctest: nova_deck_controller_library_smoke and nova_deck_qt_shell_smoke passed.
• Private Steam Deck Game Mode smoke previously passed for this branch: Deck shell launched at 1280x800, selected host/game affordances rendered, copy-preview wording stayed non-executable, and controller-style focus/navigation changed visible UI.

Notes

The Deck shell now consumes a read-only snapshot-shaped data boundary and keeps the launch/streaming boundary explicit in UI copy. Launch execution and backend/runtime integration should remain separate future work.