Fix UI: navbar layout, search form, theme switcher, content centering

- Wrap search in HTMX form for reliable parameter serialization
- Replace daisyUI data-choose-theme with custom JS (localStorage persistence)
- Restructure navbar: logo+tabs left, search center, theme+settings right
- Center releases page with max-w-4xl constrained layout
- Center artist page content
- Add loading spinner centering

src/main/resources/templates/releases.html

@@ -1,11 +1,11 @@
 {#include layouts/base}
 {#page-content}
-<div class="container mx-auto p-4">
-  <h2 class="text-2xl font-bold mb-4">📡 Releases</h2>
+<div class="w-full max-w-4xl mx-auto">
+  <h2 class="text-2xl font-bold mb-4 text-center">📡 Releases</h2>
   <div id="feed-entries" hx-get="/api/artists/feed-table" hx-trigger="load">
-    <span class="loading loading-spinner loading-lg"></span>
+    <span class="loading loading-spinner loading-lg block mx-auto"></span>
   </div>
-  <div class="mt-4 flex gap-4">
+  <div class="mt-6 flex justify-center gap-4">
     <a href="/feed/rss.xml" class="btn btn-primary">📥 RSS Feed</a>
     <a href="/feed/opml" class="btn btn-ghost">📋 OPML</a>
   </div>