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/HelpOverlay.svelte

@@ -1932,10 +1932,10 @@
 
 <div class="help-backdrop" role="presentation" onclick={handleBackdropClick}>
   <div class="help-overlay" role="dialog" aria-modal="true" aria-labelledby="help-title">
-    <header class="help-header">
-      <div class="help-title-wrap">
-        <span class="help-mark"><CircleHelp size={19} aria-hidden="true" /></span>
-        <div>
+    <header class="help-header unified-dialog-header">
+      <div class="help-title-wrap unified-dialog-heading">
+        <span class="help-mark unified-dialog-icon"><CircleHelp size={19} aria-hidden="true" /></span>
+        <div class="unified-dialog-text">
           <h2 id="help-title">{isGerman ? "Gitty Hilfe" : "Gitty Help"}</h2>
           <p>{isGerman ? "App-Anleitung und Git-Wissen an einem Ort" : "App guidance and Git knowledge in one place"}</p>
         </div>