feat(platform): add Windows support

[smartcmd]

Summary

• add native Windows backends for low-level hotkeys, WASAPI capture, SendInput/clipboard injection, UI Automation selection, focus/UIPI diagnostics, and mixed-DPI no-activate HUD placement
• add Windows-aware UI, tray behavior, local-model Vulkan-to-CPU fallback, app-local runtime staging, NSIS packaging, and multi-platform Tauri updater aggregation with final-byte signature verification
• document Windows 10 22H2+/Windows 11 x64 support, unsigned Authenticode boundary, representative application matrix, and deferred physical-environment checks

Verification

• Rust no-default: check + clippy -D warnings + all-target tests (284 unit, 4 snapshots, 23 provider)
• Rust default: check + clippy -D warnings + all-target tests (349 unit, 4 snapshots, 10 local-model, 23 provider)
• interactive Windows platform harness: 1/1
• frontend: build + 111 tests; release metadata: 5/5; actionlint and script syntax checks passed
• unsigned NSIS rebuilt and extracted; PE dependency closure, runtime hashes/licenses, WebView2 bootstrapper, and expected NotSigned/signtool failure verified
• fresh install, first launch, second-instance routing, overlay install, uninstall-data retention, reinstall, and second uninstall passed; cold start 886 ms, idle CPU 0.039%, private working set 130.66 MB

Deferred non-blocking matrix

• Windows 10 22H2, physical AltGr/QWERTZ, mixed-DPI multi-monitor, sleep/wake and RDP, second/Bluetooth microphone, and a real cross-integrity UIPI target
• SignPath/Authenticode remains deferred; no tag, release, nightly, or publication is created by this PR