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/CreateIssueDialog.svelte

@@ -85,7 +85,7 @@
 
 <dialog bind:this={dialog} aria-labelledby="create-issue-title" oncancel={event => { event.preventDefault(); if (!busy) onClose(); }}>
   <form onsubmit={submit}>
-    <header><CirclePlus size={20}/><div><h2 id="create-issue-title">{de ? "Neues Issue" : "New issue"}</h2><p>{source.label}</p></div><button class="dialog-close" type="button" aria-label={de ? "Schließen" : "Close"} disabled={busy} onclick={onClose}><X size={18}/></button></header>
+    <header class="unified-dialog-header"><span class="unified-dialog-icon" aria-hidden="true"><CirclePlus size={20}/></span><div class="unified-dialog-text"><h2 id="create-issue-title">{de ? "Neues Issue" : "New issue"}</h2><p>{source.label}</p></div><button class="dialog-close" type="button" aria-label={de ? "Schließen" : "Close"} disabled={busy} onclick={onClose}><X size={18}/></button></header>
     <div class="body">
       <div class="field"><span>{azure ? (de ? "Projekt" : "Project") : "Repository"}</span>
         <SelectMenu value={repository} options={targets} showSelectedGroup searchable disabled={loading || busy} ariaLabel={azure ? (de ? "Projekt" : "Project") : "Repository"} placeholder={loading ? (de ? "Wird geladen …" : "Loading …") : (de ? "Bitte auswählen" : "Select an option")} searchPlaceholder={de ? "Suchen …" : "Search …"} onChange={value => void selectTarget(value)}/>