cleanup: mergeDesc + versionInfo reuse .card-intro — typography consistency

Two more inline-styled secondary paragraphs converted to the shared .card-intro class:

- mergeDesc (Merge tab's intro paragraph): was color/margin-bottom:15px inline
- versionInfo (Updates card's "Version: v4.1.13" line): was color/margin-bottom:10px inline

Both sit below an h3 inside a settings-card — the same structural role as the 7 other .card-intro paragraphs. Converting unifies font-size (was browser-default ~14-16px, now the consistent 13px secondary-text size) and margin rhythm (12px) with the rest of the app's card intros. Visual delta is sub-perceptible (a few px in margin, ~1-3px in font-size) but the markup now reads its intent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.html

@@ -389,7 +389,7 @@
                     <div class="merge-container">
                         <div class="settings-card">
                             <h3 id="mergeTitle">Videos zusammenfugen</h3>
-                            <p style="color: var(--text-secondary); margin-bottom: 15px;" id="mergeDesc">
+                            <p id="mergeDesc" class="card-intro">
                                 Wahle mehrere Videos aus um sie zu einem Video zusammenzufugen.
                                 Die Reihenfolge kann per Drag & Drop geandert werden.
                             </p>
@@ -660,7 +660,7 @@
 
                     <div class="settings-card">
                         <h3 id="updateTitle">Updates</h3>
-                        <p id="versionInfo" style="margin-bottom: 10px; color: var(--text-secondary);">Version: v4.1.13</p>
+                        <p id="versionInfo" class="card-intro">Version: v4.1.13</p>
                         <button class="btn-secondary" id="checkUpdateBtn" onclick="checkUpdate()">Nach Updates suchen</button>
                     </div>