feat(plugins): resizable collapsible detail sidebar, auto-init on refresh

- Add email-detail-sidebar slot type for right-side plugin panels
- Plugin sidebar inside EmailViewer with ResizeHandle (200-500px, drag/dbl-click reset)
- Collapse/expand toggle with PanelRightClose/PanelRightOpen icons
- Call initializePlugins() on mount so plugins survive page refresh
- Fix plugin loader: call exposePluginExternals() before loading
- Fix CSP: add blob: to script-src for plugin bundle loading
- Fix enablePlugin: wire slot registration bridge before loading
- Fix email-banner PluginSlot: pass email via extraProps
- Fix sidebar-widget PluginSlot: move inside scrollable area
- Emit emailHooks.onEmailOpen/onEmailClose from selectEmail()

components/layout/sidebar.tsx

@@ -657,9 +657,9 @@ export function Sidebar({
             )}
           </>
         )}
-      </div>
 
-      <PluginSlot name="sidebar-widget" />
+        {!isCollapsed && <PluginSlot name="sidebar-widget" className="border-t border-border" />}
+      </div>
 
       {/* Compose Button */}
       <div className={cn("border-t border-border", isCollapsed ? "flex justify-center py-3" : "px-3 py-3")}>