feat(todo): edit the file right in the TODO preview (#1129)

The TODO preview is an editor on the file, as in IntelliJ: a CodeMirror view attached to the real document, so edits share the buffer with editor tabs and Save All saves them; no editing switch. The caret lands on the selected item, also when another item of the same file is selected. Read-only files fall back to the read-only preview. Dirty buffers are held rather than released (releasing would auto-save or drop the edits), and previewText reads the held text.

Co-authored-by: qtvbwfn <[email protected]>

src/features/todo/TodoView.module.css

@@ -107,3 +107,15 @@
   flex: 1;
   min-height: 0;
 }
+.previewEditor,
+.previewError {
+  flex: 1;
+  min-height: 0;
+}
+.previewError {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--fg-faint);
+  font-family: var(--font-ui);
+}