feat(deck): add product-readiness preview backend

[papi-ux]

Summary

• Isolates the Steam Deck product-readiness lane from the Android theme and drawer PR.
• Adds backend-owned read-only DTO previews, no-network product state smoke routes, guarded Deck frontend smoke scripts, and VAAPI/QSG presentation proof coverage.
• Keeps host discovery, pairing, credential access, app launch, and real stream start disabled behind guardrails.

Verification

• cmake -S clients/deck -B build/deck-pr -G Ninja
• cmake --build build/deck-pr -j 1
• ctest --test-dir build/deck-pr --output-on-failure
• git diff --check origin/master
• python3 clients/deck/scripts/deck_frontend_smoke.py --dry-run --local-artifacts build/deck-frontend-smoke-artifacts

Boundary notes

• Android app and theme paths are excluded from this branch.
• Main worktree docs/spikes artifacts were left untracked and are not part of this PR. The tracked Deck T14 spike under clients/deck/spikes remains included because it is part of the Deck source lane.
• Real Deck Game Mode smoke was not executed from this split run; the local route test and dry-run prove the command shape without starting lab UI or network sync.