phase29(app): apply DashboardDensity to Dashboard layout

[MarsDoge]

Summary

• Makes the app-owned DashboardDensity preference drive the ModernSetupApp Dashboard layout.
• Keeps Comfortable as the existing spacious layout and makes Compact reduce the top summary area plus quick-card spacing.
• Routes Dashboard keyboard navigation through the same density-aware grid used by rendering, preventing visual/selection geometry drift.
• Adds a Phase29 smoke guard for the density-aware layout contract and no HII/ConfigAccess/SetVariable regression.

Validation

• PASS python3 Tests/Smoke/smoke_validate.py
• PASS git diff --check main..HEAD
• PASS ARCH=X64 Scripts/build-modern-app.sh
• PASS WORKSPACE=/home/qdy/ModernSetupPkg/External/edk2 ARCH=LOONGARCH64 GCC_LOONGARCH64_PREFIX=loongarch64-linux-gnu- JOBS=12 Scripts/build-modern-app.sh
• PASS Claude Code final diff review
• PASS QEMU OVMF X64 screenshot capture: Dashboard Compact layout, no #PF/ASSERT/crash observed

Screenshot

• Telegram message_id: 549
• Local capture: /tmp/modernsetup-qemu/phase29-dashboard-compact.png