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/FileHistoryDialog.svelte

@@ -46,9 +46,9 @@
 
 <div class="dialog-backdrop app-chrome-backdrop file-history-dialog-backdrop" role="presentation" onclick={closeFromBackdrop}>
   <div class="file-history-dialog" role="dialog" aria-modal="true" aria-labelledby="file-history-dialog-title" tabindex="-1">
-    <header class="file-history-dialog-header">
-      <div class="file-history-dialog-icon" aria-hidden="true"><History size={19} /></div>
-      <div class="file-history-dialog-heading">
+    <header class="file-history-dialog-header unified-dialog-header">
+      <div class="file-history-dialog-icon unified-dialog-icon" aria-hidden="true"><History size={19} /></div>
+      <div class="file-history-dialog-heading unified-dialog-text">
         <span class="eyebrow">File history</span>
         <h2 id="file-history-dialog-title">{fileName(filePath)}</h2>
         <span class="file-history-dialog-path" title={filePath}>{filePath}</span>