fix(desktop): add custom window menu bar

[janburzinski]

Description

• adds a custom in window menu bar for windwos/linux
• reuses native electron meun definitions via rpc
• keeps macos unchanged
• ensures menu stays accessible when sidebar is collapsed

Screenshot/Recording (if applicable)

https://streamable.com/j4xi7v

Checklist

• I kept this PR small and focused
• I ran a self-review before opening this PR
• I ran the relevant local checks or explained why not
• I updated docs when behavior or setup changed
• I added or updated tests when behavior changed, or explained why not
• I only added comments where the logic is not obvious
• I used Conventional Commits for commit
  messages and, when possible, the PR title

[greptile-apps]

Greptile Summary

This PR adds a custom in-window app menu for desktop platforms. The main changes are:

• Shared native Electron submenu builders for File, Edit, View, and Help.
• A new app RPC path for opening a selected menu at renderer-provided coordinates.
• A Windows/Linux WindowMenuBar component that stays hidden on macOS.
• Sidebar and titlebar placement so the menu stays reachable when the sidebar is collapsed.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
apps/emdash-desktop/src/main/app/menu.ts	Factors the app menu into reusable submenu builders and adds native popup support for the custom menu bar.
apps/emdash-desktop/src/main/core/app/controller.ts	Adds a structured RPC handler for opening app menus from the renderer.
apps/emdash-desktop/src/main/core/app/service.ts	Delegates custom menu popup requests to the main menu module.
apps/emdash-desktop/src/renderer/features/sidebar/sidebar-space.tsx	Shows the custom menu bar in the sidebar header only while the sidebar is open.
apps/emdash-desktop/src/renderer/lib/components/titlebar/Titlebar.tsx	Shows the custom menu bar in the titlebar when the sidebar is collapsed.
apps/emdash-desktop/src/renderer/lib/components/titlebar/window-menu-bar.tsx	Adds the Windows/Linux menu bar buttons and opens native submenus through RPC.
apps/emdash-desktop/src/shared/events/appEvents.ts	Adds the shared menu id type used by the renderer and main process.

_{Reviews (2): Last reviewed commit: "fix(desktop): harden custom app menu" | Re-trigger Greptile}

[janburzinski]

@greptileai