feat: add standalone Rust/Tauri desktop client

[lgcyaxi]

Summary

Add a standalone Rust + Tauri 2 + React client under ahakey-desktop/, with a dedicated Windows CI workflow. Existing Swift, Java, Python and BLE bridge implementations are unchanged in this PR.

• Native BLE, Windows vendor-HID device information and capability-gated routing controls.
• Four configurable application profiles and physical keys, voice-key handling, tray controls and target-monitor captions.
• Optional local SenseVoice recognition and Doubao streaming recognition; model weights and credentials are not included.
• Display/image preview and extensible usage cards. On-device image/card upload is not implemented.

Scope

Only ahakey-desktop/** and .github/workflows/rust-client.yml are included, on top of current main. No firmware source, custom HEX, private operational documents, signing material or generated application binaries are committed.

This is a draft for architectural and integration feedback, not a replacement for the current clients.

Validation

• Windows Rust client: 51 tests passed, one controlled F18 hardware-injection test intentionally skipped.
• BLE/USB crate: 13 tests passed using the client's locked dependency graph.
• Clean-checkout frontend: 18 tests passed; TypeScript and production Vite build passed.
• Rust formatting and strict Clippy passed.
• Windows 1.1.0 signed portable build was previously compiled and its detached signature and downloaded payload hashes verified. The Rust subtree in this PR is identical to that source; the new workflow adds independent CI coverage.

Fork-side Windows CI passed: https://github.com/lgcyaxi/AhakeyAI/actions/runs/34521539792. Upstream workflow approval remains under maintainer control.

Known limitations and review gates

• A USB telemetry timeout has been reported on the installed Windows client while Windows still enumerates the keyboard. An exclusive read-only device test also timed out after opening the interface and sending its query; hardware validation remains pending. This PR does not claim that regression is fixed; HID enumeration is not treated as proof of successful telemetry.
• macOS/Linux adapters require native-host build, permissions and hardware acceptance; no cross-platform binary support is claimed.
• External WeChat/Windows dictation uses toggle shortcuts without authoritative recording-state feedback. Rapid-press synchronization remains a limitation.
• Dual-BLE/USB routing controls require compatible firmware and further multihost sleep/wake testing. No firmware is distributed with this PR.
• Microphone/provider-account, full native UI/hotplug and first-run hook acceptance are not covered by the unattended CI job.

Please advise whether this isolated client direction fits the project before we mark the PR ready for review.