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/MergeBranchDialog.svelte

@@ -63,10 +63,10 @@
 
 <div class="dialog-backdrop app-chrome-backdrop" role="presentation">
   <div class="dialog merge-branch-dialog" role="dialog" aria-modal="true" aria-labelledby="merge-branch-title" tabindex="-1">
-    <header class="dialog-header merge-branch-header">
-      <div class="merge-branch-heading">
-        <span class="merge-branch-icon" aria-hidden="true"><GitMerge size={18} /></span>
-        <div>
+    <header class="dialog-header merge-branch-header unified-dialog-header">
+      <div class="merge-branch-heading unified-dialog-heading">
+        <span class="merge-branch-icon unified-dialog-icon" aria-hidden="true"><GitMerge size={18} /></span>
+        <div class="unified-dialog-text">
           <span class="eyebrow">{isGerman ? "Branches zusammenführen" : "Combine branches"}</span>
           <h2 id="merge-branch-title">{isGerman ? "Merge konfigurieren" : "Configure merge"}</h2>
         </div>