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/CreateReviewDialog.svelte

@@ -125,7 +125,7 @@
 
 <dialog bind:this={dialog} aria-labelledby="create-review-title" oncancel={(event) => { event.preventDefault(); if (!busy && !generating) onClose(); }} onclick={(event) => { if (event.target === dialog && !busy && !generating) { const rect = dialog.getBoundingClientRect(); if (event.clientX < rect.left || event.clientX > rect.right || event.clientY < rect.top || event.clientY > rect.bottom) onClose(); } }}>
   <form onsubmit={submit}>
-    <header><div class="heading-icon"><GitPullRequest size={19} /></div><div><h2 id="create-review-title">{heading}</h2><p>{source.label}</p></div><button data-dialog-close class="close" type="button" aria-label={de ? "Schließen" : "Close"} disabled={generating || busy} onclick={onClose}><X size={18}/></button></header>
+    <header class="unified-dialog-header"><div class="heading-icon unified-dialog-icon"><GitPullRequest size={19} /></div><div class="unified-dialog-text"><h2 id="create-review-title">{heading}</h2><p>{source.label}</p></div><button data-dialog-close class="close" type="button" aria-label={de ? "Schließen" : "Close"} disabled={generating || busy} onclick={onClose}><X size={18}/></button></header>
     <div class="body">
       {#if error}<div class="error" role="alert">{error}{#if !repositories.length && !loading}<button type="button" onclick={loadRepositories}>{de ? "Erneut laden" : "Retry"}</button>{/if}</div>{/if}
       <div class="repository-field"><div class="field-heading"><span>Repository</span><small>{loading ? "…" : `${repositories.length} ${de ? "verfügbar" : "available"}`}</small></div>