refactor(surface): drop the deprecated main-tab aliases and dead diagram surface

MainTab/activeMainTab/setActiveMainTab/setMainTabGuard were deprecated
mirrors of the surface names — every call site now uses
activeSurface/setActiveSurface/setSurfaceGuard directly and the aliases
are gone, including the persisted mirror field.

The 'diagram' surface had no way to open it (navigateToDiagram had no
callers except a .drawio attachment click that navigated to a surface
nothing rendered); the surface, DiagramView, and its store plumbing are
removed, and a .drawio attachment now opens in the file panel.

?tab= deep links map to the matching context-panel surface instead of
setting a main-area surface nothing renders, and a persisted non-chat
surface can no longer rehydrate into a blank main area.

packages/ui/src/lib/addSelectionToChat.ts

@@ -151,7 +151,7 @@ export const captureSelectionMarkdownForChat = (): string | null => {
 export const addSelectionToChat = (): boolean => {
   const markdown = captureSelectionMarkdownForChat();
 
-  useUIStore.getState().setActiveMainTab('chat');
+  useUIStore.getState().setActiveSurface('chat');
   useUIStore.getState().setSessionSwitcherOpen(false);
 
   if (markdown) {