fix(ssr-overlay): render admin-sidebar gutter on the server so logged-in reload doesn't shift

Follow-up to the anti-flicker overlay: with the overlay in place, an authenticated user's hard reload
still showed the content visibly jump right by the admin-sidebar width at the moment the snapshot was
removed. Root cause is independent of the overlay (the overlay just makes it a visible "reveal"):

RootComponent computes the @slideSidebarPadding (outer-wrapper padding-left) from
`cssService.getVariable('--ds-admin-sidebar-fixed-element-width')`, but that store is only populated in
the browser (AppComponent.storeCSSVariables -> getComputedStyle). On the server the variable never
resolves, `skipWhile(!val)` blocks forever, and the SSR HTML renders `outer-wrapper { padding-left: 0 }`.
The browser then resolves the real width and applies `padding-left: 55px`, so the whole authenticated
page (and the SSR snapshot, which is just that server HTML) jumps right by the sidebar width on reveal.

Fix: on the server, fall back to the compiled default sidebar width (see $ds-admin-sidebar-* in
src/styles/_bootstrap_variables.scss) so the SSR layout already reserves the gutter; in the browser keep
waiting for the real, theme-overridable value. Non-admins are unaffected (state stays 'hidden' -> 0).

Verified locally (authenticated admin, real UA, throttled hard reload):
- SSR HTML outer-wrapper inline style is now `padding-left:55px` (was `padding-left:0`).
- snapshot vs settled `#main-content` x = 55px in both -> horizontal shift at reveal = 0px (was 55px).

Refs: dspace-customers#725, PR #1321

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: milanmajchrak

src/app/root/root.component.ts

@@ -1,5 +1,6 @@
 import { first, map, skipWhile, startWith } from 'rxjs/operators';
-import { Component, Input, OnInit } from '@angular/core';
+import { Component, Inject, Input, OnInit, PLATFORM_ID } from '@angular/core';
+import { isPlatformServer } from '@angular/common';
 import { Router } from '@angular/router';
 
 import { combineLatest as combineLatestObservable, Observable, of } from 'rxjs';
@@ -42,20 +43,29 @@ export class RootComponent implements OnInit {
     private router: Router,
     private cssService: CSSVariableService,
     private menuService: MenuService,
-    private windowService: HostWindowService
+    private windowService: HostWindowService,
+    @Inject(PLATFORM_ID) private platformId: any,
   ) {
     this.notificationOptions = environment.notifications;
   }
 
   ngOnInit() {
     this.isSidebarVisible$ = this.menuService.isMenuVisibleWithVisibleSections(MenuID.ADMIN);
 
+    // The CSS-variable store is only populated in the browser (from getComputedStyle in
+    // AppComponent.storeCSSVariables). On the server it stays empty, so skipWhile(!val) would block
+    // forever and the @slideSidebarPadding animation renders the outer-wrapper with padding-left:0.
+    // The browser then resolves the real width and applies it, so for an authenticated user the whole
+    // page (and the anti-flicker SSR snapshot, which is just the server HTML) visibly jumps right by
+    // the sidebar width once the overlay is removed. On the server, fall back to the compiled default
+    // width (see $ds-admin-sidebar-* in src/styles/_bootstrap_variables.scss) so the SSR layout already
+    // reserves the gutter and matches the CSR; in the browser keep waiting for the real, theme-overridable value.
     this.expandedSidebarWidth$ = this.cssService.getVariable('--ds-admin-sidebar-total-width').pipe(
-      skipWhile((val) => !val),
+      isPlatformServer(this.platformId) ? map((val) => val || '305px') : skipWhile((val) => !val),
       first(),
     );
     this.collapsedSidebarWidth$ = this.cssService.getVariable('--ds-admin-sidebar-fixed-element-width').pipe(
-      skipWhile((val) => !val),
+      isPlatformServer(this.platformId) ? map((val) => val || '55px') : skipWhile((val) => !val),
       first(),
     );