Roll back native Help Viewer integration; fix duplicate View menu

Per Rune: the NSHelpManager-based Cmd+? fix from the last few commits
technically worked (registration was correct) but opened Apple's
broken generic Tips landing page instead of oAI's own content on the
macOS 27 beta this is built against — worse than the original
browser-tab behavior. Revisit at macOS 27 RC1 (see CLAUDE.md).

- openHelp() reverts to NSWorkspace.open() on index.html directly.
- Removed the "In-App Help" Ctrl+Cmd+H menu item entirely — HelpView's
  panel (search already fully working) is reachable only via /help
  from the input field now, by design.
- Renamed the custom CommandMenu("View") to CommandMenu("Chat") — it
  was colliding with the "View" menu macOS auto-adds for
  NavigationSplitView (Enter Full Screen, etc.), producing two
  identically-titled top-level menus in the menu bar.

oAI/Resources/oAI.help/Contents/Resources/en.lproj/index.html

@@ -1526,9 +1526,6 @@ Whenever the user asks you to translate something, translate it to Norwegian Bok
                     <dt><kbd>⌘,</kbd></dt>
                     <dd>Open Settings</dd>
 
-                    <dt><kbd>⌃⌘H</kbd></dt>
-                    <dd>Show in-app Help</dd>
-
                     <dt><kbd>⌘?</kbd></dt>
                     <dd>Open this Help (macOS Help)</dd>