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/RenameBranchDialog.svelte

@@ -50,8 +50,9 @@
   role="presentation"
 >
   <div class="dialog rename-branch-dialog" role="dialog" aria-modal="true" aria-label={branch.remote ? "Rename remote branch" : "Rename branch"} tabindex="-1">
-    <header class="dialog-header">
-      <div>
+    <header class="dialog-header unified-dialog-header">
+      <span class="unified-dialog-icon" aria-hidden="true"><GitBranch size={18} /></span>
+      <div class="unified-dialog-text">
         <span class="eyebrow">{branch.remote ? "Rename remote branch" : "Rename branch"}</span>
         <h2 class="mt-0.5 text-ink text-base font-bold leading-tight">{branch.name}</h2>
       </div>