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/GitLfsDialog.svelte

@@ -91,10 +91,10 @@
 
 <div class="dialog-backdrop" role="presentation">
   <div class="dialog lfs-dialog" role="dialog" aria-modal="true" aria-labelledby="lfs-dialog-title">
-    <header class="dialog-header lfs-dialog-header">
-      <div class="lfs-heading">
-        <span class="lfs-mark" aria-hidden="true"><Box size={19} /></span>
-        <div>
+    <header class="dialog-header lfs-dialog-header unified-dialog-header">
+      <div class="lfs-heading unified-dialog-heading">
+        <span class="lfs-mark unified-dialog-icon" aria-hidden="true"><Box size={19} /></span>
+        <div class="unified-dialog-text">
           <span class="eyebrow">Large file storage</span>
           <p class="dialog-title" id="lfs-dialog-title">Git LFS</p>
         </div>