Add 'Reveal in Finder/Explorer' action to the project context menu

[Ron537]

Summary

Add a "Reveal in Finder" (macOS) / "Reveal in Explorer" (Windows) / "Open in File Manager" (Linux) action to the right-click context menu of project rows.

Acceptance criteria

• New IPC channel exposed via preload: window.dplex.shell.revealInFolder(absPath: string): Promise<void>.
• Main-process handler calls Electron's shell.showItemInFolder(absPath).
• Menu item added to the right-click menu in src/renderer/src/components/projects/ProjectItem.tsx (look for the existing "Open Terminal" and "Copy Path" entries).
• Label adapts to platform: "Reveal in Finder" on macOS, "Reveal in Explorer" on Windows, "Open in File Manager" elsewhere.

Files to touch

• src/preload/index.ts (new IPC contract)
• src/main/index.ts (IPC handler)
• src/renderer/src/components/projects/ProjectItem.tsx (menu item)
• tests/unit/<existing-relevant-test>.ts (verify IPC wiring)

How to test

• Run npm run dev, right-click any project, select the new menu item — the system file manager should open at the project root with the folder pre-selected.
• Repeat across all three OSes if you can.

Notes

• shell.showItemInFolder is built into Electron — no new dependencies.
• Cross-platform string for the label is required per repo conventions.

Effort: 2-4 hours.