feat: Windows desktop support + no console flashes

[XiaoLrrr]

Summary

Adds first-class Windows desktop support to the Tauri app and fixes a console-window flashing issue that only shows up when running the desktop app on Windows.

Changes

1. Windows desktop window behavior

• Add crates/hk-desktop/tauri.windows.conf.json so Windows builds don't inherit the macOS-only window styling (transparent: true, sidebar vibrancy, overlay title bar). On Windows this previously rendered a transparent window that visually overlapped other apps and flickered while dragging.
• Make the close-button behavior platform-aware in main.rs: macOS keeps hide-on-close (dock), Windows/Linux quit on close instead of leaving a hidden background process.

2. No more console window flashes

• Add hk_core::command() (crates/hk-core/src/process.rs) which sets CREATE_NO_WINDOW on Windows.
• Route all subprocess spawns through it (git / where / npm probing in scanner, manager, service, skills_cli, and the desktop extensions/install commands). Previously each probe of a CLI binary (where wecom-cli, …) flashed a console window when launched from the GUI process.

Testing

• Built the Windows desktop app locally (cargo tauri build --no-bundle) and verified the window renders with a standard title bar and no transparent overlap.
• Verified at runtime that where.exe probes spawned during scanning no longer create conhost.exe console windows.