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/HistoryPanel.svelte

@@ -1167,8 +1167,9 @@
       aria-modal="true"
       aria-label="Select visible branches"
     >
-      <header class="branch-filter-dialog-head">
-        <div>
+      <header class="branch-filter-dialog-head unified-dialog-header">
+      <span class="unified-dialog-icon" aria-hidden="true"><GitBranch size={18} /></span>
+        <div class="unified-dialog-text">
           <span class="eyebrow">Git graph</span>
           <h3>Visible branches</h3>
         </div>