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/AiCommitSplitDialog.svelte

@@ -46,8 +46,9 @@
 
 <div class="dialog-backdrop" role="presentation">
   <div class="dialog split-dialog" role="dialog" aria-modal="true" aria-label="AI commit split">
-    <header class="dialog-header">
-      <div><span class="eyebrow">Staged changes</span><h2>Split into logical commits</h2></div>
+    <header class="dialog-header unified-dialog-header">
+      <span class="unified-dialog-icon" aria-hidden="true"><GitCommitHorizontal size={18} /></span>
+      <div class="unified-dialog-text"><span class="eyebrow">Staged changes</span><h2>Split into logical commits</h2></div>
       <button class="dialog-close" type="button" onclick={onClose} disabled={isApplying} aria-label="Close"><X size={18} /></button>
     </header>
     <div class="split-intro"><Sparkles size={18} /><p>{draft.summary}</p></div>