feat(ui): add consistent page-header layout and styles

Introduce a shared page-header/page-heading pattern and CSS to
standardize headers across top-level sections. Components were
updated to adopt the new classes and slightly smaller icon sizing
for a more compact, consistent header appearance. This improves
visual consistency and makes future header adjustments simpler.

- Add reusable page-header styles for spacing, borders, and bg
- Standardize icon sizing and heading layout across screens

src/lib/components/ReviewCenter.svelte

@@ -402,8 +402,8 @@
 {/if}
 
 <section class:has-inspector={detailOpen && !!selected} class="review-center" aria-label="Review Center">
-  <header class="review-header">
-    <div class="review-heading"><GitPullRequest size={17} /><h1>Review Center</h1></div>
+  <header class="review-header page-header">
+    <div class="review-heading page-heading"><GitPullRequest size={17} /><h1>Review Center</h1></div>
     {#if activeSource}<button class="create-review" type="button" disabled={loading} onclick={() => { detailOpen = false; createOpen = true; }}><GitPullRequest size={14}/>{activeSource.provider.startsWith("gitlab") ? (de ? "MR erstellen" : "Create MR") : (de ? "PR erstellen" : "Create PR")}</button>{/if}
   </header>