Added Mod+N as another shortcut for opening a new tab

Fixes

• CoreStopped subagents from launching nested subagents by default, with a configurable subagent_depth limit when needed.
• CoreImproved default reasoning depth for Meta models.
• DesktopRestored the Help button in release builds.
• DesktopKept sessions with null archive times visible instead of dropping them from the home list.
• DesktopHid the drawer close button on Windows where it conflicts with the window chrome.