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/LinePatchDialog.svelte

@@ -297,8 +297,8 @@
 
 <div class="dialog-backdrop app-chrome-backdrop" role="presentation">
   <div class="dialog line-patch-dialog" role="dialog" aria-modal="true" aria-label={t("Geänderte Zeilen", "Changed lines")}>
-    <header class="dialog-header">
-      <div class="patch-identity"><FileDiff size={23} aria-hidden="true" /><div><p class="dialog-title" title={displayPath}>{displayPath}</p><span class="patch-scope">{scopeLabel}</span></div></div>
+    <header class="dialog-header unified-dialog-header">
+      <div class="patch-identity unified-dialog-heading"><span class="unified-dialog-icon" aria-hidden="true"><FileDiff size={23} aria-hidden="true" /></span><div class="unified-dialog-text"><p class="dialog-title" title={displayPath}>{displayPath}</p><span class="patch-scope">{scopeLabel}</span></div></div>
       <div class="dialog-header-actions">
         <button class="external-diff" type="button" onclick={onExternalDiff} disabled={isBusy || isLoading} title={t(`In ${diffName} öffnen`, `Open in ${diffName}`)}><span>{t("Extern öffnen", "Open externally")}</span><ExternalLink size={14} /><span>·</span><span>{diffName}</span></button>
         <button class="icon-action" type="button" onclick={onRefresh} disabled={isBusy || isLoading} aria-label={t("Aktualisieren", "Refresh")} title={t("Aktualisieren", "Refresh")}><RefreshCw size={16} /></button>