feat: Add BackgroundSize dropdown to Edit view

Views/Slides/Edit.cshtml

@@ -62,6 +62,17 @@
                         <button type="button" class="btn btn-sm btn-outline-secondary mt-1" onclick="uploadBackgroundImage()"><i class="bi bi-upload me-1"></i>Upload</button>
                         <input type="file" id="bgImageUpload" class="d-none" accept="image/*" />
                     </div>
+                    <div class="mb-3">
+                        <label asp-for="BackgroundSize" class="form-label">Image Sizing</label>
+                        <select asp-for="BackgroundSize" class="form-select">
+                            <option value="cover">Cover — fill entire slide, crop if needed</option>
+                            <option value="contain">Contain — fit whole image, may show background</option>
+                            <option value="fill">Fill — stretch to fit exactly</option>
+                            <option value="scale-down">Scale Down — shrink to fit, never enlarge</option>
+                            <option value="auto">Auto — original size</option>
+                        </select>
+                        <div class="form-text">How the background image fills the slide area.</div>
+                    </div>
                     <div class="mb-3">
                         <label asp-for="CustomCss" class="form-label">Custom CSS</label>
                         <textarea asp-for="CustomCss" class="form-control font-monospace" rows="4"></textarea>