Phase 4: Background Removal (Transformers.js)

- Added @xenova/transformers dependency
- useBackgroundRemoval hook with RMBG-1.4 model
- Client-side background removal with progress indicator
- Background removal button in properties panel (image elements only)
- ~170MB model cached after first download

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

client/src/index.css

@@ -490,6 +490,54 @@ input, textarea, select {
   color: white;
 }
 
+.bg-removal-container {
+  margin-top: 1rem;
+  padding-top: 1rem;
+  border-top: 1px solid var(--border);
+}
+
+.bg-removal-btn {
+  width: 100%;
+  padding: 0.75rem;
+  background: linear-gradient(135deg, #8b5cf6, #ec4899);
+  color: white;
+  border: none;
+  border-radius: var(--radius-md);
+  font-weight: 500;
+  cursor: pointer;
+  transition: all 0.2s;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 0.5rem;
+}
+
+.bg-removal-btn:hover:not(:disabled) {
+  transform: translateY(-1px);
+  box-shadow: var(--shadow-md);
+}
+
+.bg-removal-btn:disabled {
+  opacity: 0.7;
+  cursor: not-allowed;
+}
+
+.bg-removal-hint {
+  font-size: 0.7rem;
+  color: var(--text-muted);
+  margin: 0.5rem 0 0 0;
+  line-height: 1.4;
+}
+
+.spinner-small {
+  width: 16px;
+  height: 16px;
+  border: 2px solid rgba(255, 255, 255, 0.3);
+  border-top-color: white;
+  border-radius: 50%;
+  animation: spin 1s linear infinite;
+}
+
 /* Responsive */
 @media (max-width: 900px) {
   .app-layout {