feat: add sheet max-height and background color controls

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

admin/branding.php

@@ -391,6 +391,19 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['asset'])) {
                 <div class="hint"><?= $s['hint'] ?></div>
             </div>
             <?php endforeach; ?>
+            <div class="field">
+                <label>Max Height</label>
+                <div class="range-row">
+                    <input type="range" name="theme[sheet_max_height]" value="<?= $theme['sheet_max_height'] ?? '75' ?>" min="40" max="95" oninput="this.nextElementSibling.textContent=this.value+'vh'">
+                    <span class="range-val"><?= $theme['sheet_max_height'] ?? '75' ?>vh</span>
+                </div>
+                <div class="hint">Sheet max viewport height</div>
+            </div>
+            <div class="field">
+                <label>Sheet Background</label>
+                <input type="color" name="theme[sheet_bg]" value="<?= $theme['sheet_bg'] ?? '#0f0f1e' ?>" class="color-input">
+                <div class="hint">Bottom sheet bg color</div>
+            </div>
         </div>
     </div>