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/GlobalSearchDialog.svelte

@@ -137,8 +137,9 @@
   role="presentation"
 >
   <div class="dialog global-search-dialog" role="dialog" aria-modal="true" aria-label="Global search" tabindex="-1">
-    <header class="dialog-header">
-      <div>
+    <header class="dialog-header unified-dialog-header">
+      <span class="unified-dialog-icon" aria-hidden="true"><Search size={18} /></span>
+      <div class="unified-dialog-text">
         <span class="eyebrow">Global search</span>
         <h2 class="dialog-title">{activeTab === "code" ? "Find where code was introduced" : "Find file history"}</h2>
       </div>