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>

src/core/todo-transformer.ts

@@ -65,6 +65,78 @@ export function insertTodoAtEnd(content: string, todoLine: string): string {
 	return content + '\n' + todoLine;
 }
 
+/**
+ * Move a todo (and its children) from one position to another within the same content.
+ * @param content - The file content
+ * @param todoLineNumber - Line number of the todo to move
+ * @param childLineNumbers - Line numbers of child todos (must be consecutive after todoLineNumber)
+ * @param targetLineNumber - Line number to insert before (0-based). Lines are inserted at this position.
+ */
+export function moveTodoWithChildren(
+	content: string,
+	todoLineNumber: number,
+	childLineNumbers: number[],
+	targetLineNumber: number
+): string {
+	const lines = content.split('\n');
+	const allLineNumbers = [todoLineNumber, ...childLineNumbers].sort((a, b) => a - b);
+
+	// Extract the lines to move
+	const movedLines = allLineNumbers.map((ln) => lines[ln]!);
+
+	// If moving to same position, no-op
+	if (targetLineNumber === todoLineNumber) {
+		return content;
+	}
+
+	// Remove lines from original positions (in reverse to preserve indices)
+	for (let i = allLineNumbers.length - 1; i >= 0; i--) {
+		lines.splice(allLineNumbers[i]!, 1);
+	}
+
+	// Adjust target position for removed lines
+	let adjustedTarget = targetLineNumber;
+	for (const ln of allLineNumbers) {
+		if (ln < targetLineNumber) {
+			adjustedTarget--;
+		}
+	}
+
+	// Insert at adjusted target
+	lines.splice(adjustedTarget, 0, ...movedLines);
+
+	return lines.join('\n');
+}
+
+/**
+ * Move a todo (and its children) under a heading within the same file.
+ * Inserts at the end of the heading's section.
+ * @param content - The file content
+ * @param todoLineNumber - Line number of the todo to move
+ * @param childLineNumbers - Line numbers of child todos
+ * @param headingLineNumber - Line number of the target heading
+ * @param nextHeadingLineNumber - Optional line number of the next heading (section boundary)
+ */
+export function moveTodoUnderHeadingInFile(
+	content: string,
+	todoLineNumber: number,
+	childLineNumbers: number[],
+	headingLineNumber: number,
+	nextHeadingLineNumber?: number
+): string {
+	// Determine insert position: end of section (before next heading) or after heading
+	let targetLineNumber: number;
+	if (nextHeadingLineNumber !== undefined) {
+		targetLineNumber = nextHeadingLineNumber;
+	} else {
+		// No next heading — insert at end of file
+		const lines = content.split('\n');
+		targetLineNumber = lines.length;
+	}
+
+	return moveTodoWithChildren(content, todoLineNumber, childLineNumbers, targetLineNumber);
+}
+
 /**
  * Insert a todo line under a specific heading.
  * @param content - The file content