Hide immutable type field in color strip source edit modal

Type is set at creation and cannot be changed, so hide the selector
when editing (same pattern already used in value source editor).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/templates/modals/css-editor.html

@@ -14,7 +14,7 @@
                     <input type="text" id="css-editor-name" data-i18n-placeholder="color_strip.name.placeholder" placeholder="Wall Strip" required>
                 </div>
 
-                <div class="form-group">
+                <div id="css-editor-type-group" class="form-group">
                     <div class="label-row">
                         <label for="css-editor-type" data-i18n="color_strip.type">Type:</label>
                         <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>