Add custom short code support to web UI

Allow users to optionally specify their own short code when
shortening a URL, instead of always generating a random one.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/index.html

@@ -13,6 +13,10 @@
             <label for="value">URL</label>
             <input type="text" id="value" name="value" placeholder="https://..." required>
         </div>
+        <div class="form-group">
+            <label for="custom">Custom short code (optional)</label>
+            <input type="text" id="custom" name="custom" placeholder="e.g. mylink" pattern="[a-zA-Z0-9]+" title="Letters and numbers only">
+        </div>
         <div class="form-group">
             <label>
                 <input type="checkbox" name="strip"> Strip query string