fix(windows): stop console windows flashing on every host spawn

[juacker]

Problem

On Windows, every host process CLAI spawns briefly flashes a console window: the startup which/where provider probes (two flashes per cold boot), editor/terminal detection, opening a file with the system default app (explorer.exe), opening an editor (.cmd shims like code.cmd), the where PATHEXT resolution, provider CLI sessions, stdio MCP servers, git for skill sources, taskkill tree-kills, and bash_exec children.

Root cause: CLAI's desktop binary is a GUI-subsystem app with no console of its own, so CreateProcessW allocates a brand-new visible console for every console-subsystem child unless the spawn passes CREATE_NO_WINDOW.

Fix

• New src-tauri/src/windows_console.rs: HideConsoleWindow trait with hide_console_window() for std::process::Command and tokio::process::Command — applies creation_flags(CREATE_NO_WINDOW) on Windows, no-op elsewhere, so call sites stay platform-unconditional.
• Applied at every background spawn site: providers::get_host_command (all which/where/version/xdg-mime probes), providers::build_host_cli_command_impl (provider CLI sessions), system_apps::resolve_windows_program's where probe, spawn_host_detached hidden paths (editors, explorer.exe/open/xdg-open), stdio MCP server spawns, git for skill sources, taskkill, and the unsupported-sandbox runner (bash_exec on Windows).
• spawn_host_detached now takes an explicit HostWindow::{Hidden,Visible}: terminal launches stay Visible (for cmd/powershell the console window is the launched terminal; wt draws its own window either way), and custom editor commands stay Visible (a console custom editor like vim would otherwise run invisibly holding the file).
• windows-sys gains the Win32_System_Threading feature for the CREATE_NO_WINDOW constant.

Verification

• cargo test --lib: 802/802 passed (incl. 2 new smoke tests covering both command types); clippy and rustfmt clean.
• Independent static review (2 rounds): round 1 production_quality with 3 minor findings, all fixed; round 2 production_quality, zero findings.
• ⚠️ #[cfg(windows)] paths are eye-verified only (dev host is Linux) — please confirm the Windows CI build, and ideally one manual smoke on Windows: cold boot (no flashes), open file/editor (no flash), open terminal (window still appears).