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/IssueCenter.svelte

@@ -172,8 +172,8 @@
 <svelte:window onkeydown={event => { if (event.key === "Escape" && selectedId) void closeDetails(); }} />
 
 <section class="issue-center" aria-label="Issues">
-  <header class="workspace-heading">
-    <div class="workspace-title"><span class="workspace-symbol"><CircleDot size={21} strokeWidth={1.7} /></span><div><h1>Issues</h1><p>{de ? "Arbeit im Blick. Über alle Integrationen." : "Work in focus. Across your integrations."}</p></div></div>
+  <header class="workspace-heading page-header">
+    <div class="page-heading"><CircleDot size={17} /><h1>Issues</h1></div>
     <button class="workspace-button" onclick={onOpenSettings} title={de ? "Integrationen konfigurieren" : "Configure integrations"}><Settings2 size={14} />{de ? "Integrationen" : "Integrations"}</button>
   </header>
   {#if !sources.length}