refactor(dialogs): add unified dialog header chrome and opt-in markup

Introduce a shared dialog header style (.unified-dialog-header) in app.css and opt dialog components into the new chrome by updating their header markup. Headers now use unified-dialog-icon and unified-dialog-text elements (and import the matching Lucide icons where needed), which standardizes icon placement, title/eyebrow layout, close button styling and responsive behavior. The Command Palette layout was adjusted to include the new header and its grid rows.

The CSS is explicitly opt-in (so page/section headers remain unchanged) and includes hover/focus styles and a theme-sensitive close color variable. This commit is a UI refactor only — no API or behavior logic changes.

src/lib/components/IssueCenter.svelte

@@ -302,8 +302,8 @@
       </div>
       {#if selected}
         <aside bind:this={detailPanel} class="issue-detail-drawer" aria-label={de ? "Issue-Details" : "Issue details"} transition:fly={{ x: 140, duration: reduceMotion ? 0 : 210, easing: cubicOut }}>
-          <header class="issue-detail-header">
-            <div><CircleDot size={17} /><strong>{source?.label} · Issue</strong></div>
+          <header class="issue-detail-header unified-dialog-header">
+            <div class="unified-dialog-heading"><span class="unified-dialog-icon" aria-hidden="true"><CircleDot size={17} /></span><div class="unified-dialog-text"><strong>{source?.label} · Issue</strong></div></div>
             <div class="issue-detail-header-actions">
               {#if selected.webUrl}<button class="workspace-button close-button" onclick={() => openIssue(selected!.webUrl)} aria-label={de ? "Im Anbieter öffnen" : "Open in provider"}><ExternalLink size={16} /></button>{/if}
               <button data-dialog-close class="workspace-button close-button" onclick={closeDetails} aria-label={de ? "Details schließen" : "Close details"}><X size={17} /></button>