Fix repeated terminal launches when reopening GodotHub with Godot running

[hanmi255]

Summary

Fixes a Windows issue where reopening GodotHub while a Godot project was
still running could cause a terminal window to repeatedly open and close.

The same session-tracking behavior is now unified across Windows and Unix
platforms, including Linux and macOS.

Problem

The issue could be reproduced as follows:

1. Start GodotHub.
2. Open a Godot project.
3. Enable Close application on project open, or close GodotHub manually.
4. Leave the Godot project running.
5. Start GodotHub again.

Under the affected implementation, GodotHub recovered the stale project session
using platform-specific process detection. The monitoring path could repeatedly
launch external process-query commands such as PowerShell, WMIC, or ps.

On Windows, these helper-process launches could become visible as terminal
windows, producing this loop:

terminal starts
→ terminal closes immediately
→ terminal starts again
→ terminal closes immediately
→ repeat

Closing the Godot process stopped the loop because the stale-session monitor no
longer had an active process to track.

Root Cause

Process discovery and process monitoring were mixed together:

• Unix used path-based polling.
• Windows used PID checks together with external process discovery.
• Recovered sessions did not share one platform-neutral process-tracking model.
• External process-query commands could be invoked repeatedly during monitoring.

Solution

This PR introduces a dedicated cross-platform process module.

Process tracking

• Process discovery is performed once during stale-session recovery.
• Recovered sessions are tracked by PID on both Windows and Unix.
• Normal monitoring uses direct PID liveness checks.
• Windows uses OpenProcess and GetExitCodeProcess.
• Unix uses kill(pid, 0).
• If a PID appears to have exited, the process identity is revalidated once
  against the stored Godot project path.
• Process termination verifies the PID/project identity before killing it.

Test organization

Process tests were moved out of projects.rs and organized as:

src-tauri/tests/
├── common/process.rs
├── windows/process.rs
└── unix/process.rs

Expected Result

• Reopening GodotHub while Godot is still running no longer starts a repeated
  terminal open/close loop.
• The normal process-monitoring loop does not repeatedly launch PowerShell,
  WMIC, or ps.
• Windows, Linux, and macOS use the same PID-based session lifecycle.
• Existing project launch, close, stale-session recovery, and time tracking
  behavior remains unchanged.

Verification

Passed locally:

• cargo test --manifest-path src-tauri/Cargo.toml
• cargo check --manifest-path src-tauri/Cargo.toml --target x86_64-pc-windows-msvc
• bun run lint
• bunx tsc --noEmit

The Windows-specific tests run on the current Windows environment. The Unix
tests are selected with cfg(unix) and should be run on Linux/macOS CI or
native development environments.

Files Changed

• src-tauri/src/process.rs
• src-tauri/src/projects.rs
• src-tauri/src/terminal.rs
• src-tauri/src/lib.rs
• src-tauri/Cargo.toml
• src-tauri/Cargo.lock
• src-tauri/tests/common/process.rs
• src-tauri/tests/windows/process.rs
• src-tauri/tests/unix/process.rs