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/CompareSelectDialog.svelte

@@ -75,10 +75,10 @@
   role="presentation"
 >
   <div class="dialog compare-select-dialog" role="dialog" aria-modal="true" aria-label={isGerman ? "Branches oder Commits vergleichen" : "Compare branches or commits"} tabindex="-1">
-    <header class="compare-dialog-head">
-      <div class="compare-dialog-title">
-        <span class="compare-dialog-mark"><GitCompare size={18} aria-hidden="true" /></span>
-        <div>
+    <header class="compare-dialog-head unified-dialog-header">
+      <div class="compare-dialog-title unified-dialog-heading">
+        <span class="compare-dialog-mark unified-dialog-icon"><GitCompare size={18} aria-hidden="true" /></span>
+        <div class="unified-dialog-text">
           <h2>{isGerman ? "Branches oder Commits vergleichen" : "Compare branches or commits"}</h2>
           <p>{isGerman ? "Zwei Repository-Stände direkt gegenüberstellen" : "Review two repository states side by side"}</p>
         </div>