fix(desktop): suppress console window flashes from background spawns on Windows

[adamentwistle]

Fixes #2490

On Windows, every std::process::Command spawn of a console-subsystem binary
(git, node, ffmpeg, agent CLIs) opens a visible console window unless the
CREATE_NO_WINDOW process-creation flag is set. The desktop app spawns these
in the background constantly — git exec, agent runtime readiness/version
probes, auth status probes, ffmpeg transcodes, provider invocations — so black
console windows flash over the UI throughout normal use.

Change

Set the CREATE_NO_WINDOW creation flag (0x0800_0000) on all background
spawn sites under desktop/src-tauri, guarded by #[cfg(windows)] via
std::os::windows::process::CommandExt:

• commands/project_git_exec.rs — run_git
• commands/agent_discovery/managed_node.rs — managed node --version probe
• commands/agent_model_process.rs — agent models listing
• commands/agent_auth.rs — ACP auth command + Claude subscription login
• commands/media_transcode.rs — ffmpeg command builder + test fixture generation
• commands/relay_reconnect.rs — run_with_timeout
• managed_agents/backend.rs — invoke_provider
• managed_agents/discovery.rs — login-shell probe, auth status probe, codex ACP version probe
• managed_agents/agent_env.rs — env probe tests

No new dependencies. No behavior change on non-Windows platforms (the flag is
compiled out). Interactive/foreground spawns are untouched.

Testing

• cargo check passes on Windows (MSVC, Rust 1.95.0) with no new warnings vs a
  clean-upstream baseline.
• Running this patch daily on a source build of v0.4.23 on Windows 11 Pro 25H2:
  the constant console-window flashes are gone; git operations, agent
  discovery, auth probes, and transcodes all still work.