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/BranchDeleteConfirmDialog.svelte

@@ -27,12 +27,12 @@
 
 <div class="dialog-backdrop" role="presentation">
   <div class="dialog branch-delete-dialog" role="dialog" aria-modal="true" aria-labelledby="branch-delete-title">
-    <header class="dialog-header branch-delete-header">
-      <div class="branch-delete-heading">
-        <span class:force class="branch-delete-heading-icon" aria-hidden="true">
+    <header class="dialog-header branch-delete-header unified-dialog-header">
+      <div class="branch-delete-heading unified-dialog-heading">
+        <span class:force class="branch-delete-heading-icon unified-dialog-icon" aria-hidden="true">
           <Trash2 size={16} />
         </span>
-        <div>
+        <div class="unified-dialog-text">
         <span class="eyebrow">{branch.remote ? "Remote branch" : force ? "Force delete" : "Delete branch"}</span>
           <p class="dialog-title" id="branch-delete-title">{title}</p>
         </div>