Add heading-grouped, nested, draggable todos with keyboard navigation

Enhance the todo tracker sidebar panel with:
- Group todos by heading from the source file
- Display nested (indented) todos as a tree
- Drag-and-drop to reorder todos within/across heading groups
- Keyboard navigation (arrow keys, Enter to open, M to move, toggle hotkey)
- Right-click context menu replacing the ellipsis button
- Proper focus management (no focus stealing on refresh)
- Platform-aware Obsidian hotkey matching for toggle checkbox

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

styles.css

@@ -2,6 +2,7 @@
 
 .todo-tracker-container {
 	padding: 10px;
+	outline: none;
 }
 
 .todo-tracker-header {
@@ -20,18 +21,45 @@
 	text-align: center;
 }
 
+/* Heading Groups */
+.todo-tracker-group {
+	margin-bottom: 12px;
+	border-radius: 4px;
+	transition: background-color 0.15s ease;
+}
+
+.todo-tracker-group-heading {
+	font-weight: 600;
+	font-size: 13px;
+	color: var(--text-muted);
+	padding: 4px 4px 4px 0;
+	margin-bottom: 2px;
+}
+
+/* Todo Lists */
 .todo-tracker-list {
 	list-style: none;
 	padding: 0;
 	margin: 0;
 }
 
+.todo-tracker-nested-list {
+	list-style: none;
+	padding-left: 20px;
+	margin: 0;
+	width: 100%;
+}
+
+/* Todo Items */
 .todo-tracker-item {
 	display: flex;
 	align-items: center;
+	flex-wrap: wrap;
 	gap: 8px;
 	padding: 6px 4px;
 	border-radius: 4px;
+	cursor: grab;
+	transition: background-color 0.15s ease;
 }
 
 .todo-tracker-item:hover {
@@ -55,23 +83,26 @@
 	color: var(--text-muted);
 }
 
-.todo-tracker-menu-btn {
-	flex-shrink: 0;
-	background: none;
-	border: none;
-	padding: 4px;
-	border-radius: 4px;
-	cursor: pointer;
-	color: var(--text-muted);
-	opacity: 0;
-	transition: opacity 0.15s ease;
-}
-
-.todo-tracker-item:hover .todo-tracker-menu-btn {
-	opacity: 1;
-}
-
-.todo-tracker-menu-btn:hover {
+/* Keyboard focus */
+.todo-tracker-item-focused {
+	outline: 2px solid var(--interactive-accent);
+	outline-offset: -2px;
+	background-color: var(--background-modifier-hover);
+}
+
+/* Drag and drop */
+.todo-tracker-item-dragging {
+	opacity: 0.4;
+}
+
+.todo-tracker-drop-above {
+	border-top: 2px solid var(--interactive-accent);
+}
+
+.todo-tracker-drop-below {
+	border-bottom: 2px solid var(--interactive-accent);
+}
+
+.todo-tracker-drag-over {
 	background-color: var(--background-modifier-hover);
-	color: var(--text-normal);
 }