feat: Implement sidebar collapse and expand functionality

- Add CSS for collapsed sidebar state and transitions
- Introduce SidebarToggle class for managing collapse/expand logic
- Integrate SidebarToggle initialization in main script
- Add toggle button to navbar and make mini sidebar clickable
- Store sidebar collapsed state in localStorage
- Filter image files and directories in view mode via FileTree
- Make navbar brand clickable to navigate to collection root or home

static/css/layout.css

@@ -124,6 +124,63 @@ body {
     height: 100%;
     overflow: hidden;
     /* Prevent pane scrolling */
+    transition: flex 0.3s ease, min-width 0.3s ease, max-width 0.3s ease;
+}
+
+/* Collapsed sidebar state - mini sidebar */
+#sidebarPane.collapsed {
+    flex: 0 0 50px;
+    min-width: 50px;
+    max-width: 50px;
+    border-right: 1px solid var(--border-color);
+    position: relative;
+    cursor: pointer;
+}
+
+/* Hide file tree content when collapsed */
+#sidebarPane.collapsed #fileTree {
+    display: none;
+}
+
+/* Hide collection selector when collapsed */
+#sidebarPane.collapsed .collection-selector {
+    display: none;
+}
+
+/* Visual indicator in the mini sidebar */
+#sidebarPane.collapsed::before {
+    content: '';
+    display: block;
+    width: 100%;
+    height: 100%;
+    background: var(--bg-secondary);
+    transition: background 0.2s ease;
+}
+
+/* Hover effect on mini sidebar */
+#sidebarPane.collapsed:hover::before {
+    background: var(--hover-bg);
+}
+
+/* Right arrow icon in the center of mini sidebar */
+#sidebarPane.collapsed::after {
+    content: '\F285';
+    /* Bootstrap icon chevron-right */
+    font-family: 'bootstrap-icons';
+    position: absolute;
+    top: 50%;
+    left: 50%;
+    transform: translate(-50%, -50%);
+    font-size: 20px;
+    color: var(--text-secondary);
+    pointer-events: none;
+    opacity: 0.5;
+    transition: opacity 0.2s ease;
+    cursor: pointer;
+}
+
+#sidebarPane.collapsed:hover::after {
+    opacity: 1;
 }
 
 #editorPane {