docs: add nested checklists to features and architecture docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -103,6 +103,21 @@ Key tables:
 - `shared_notes` - Public share links (noteId → token, 1:1 with cascade delete)
 - `sync_log` - Sync operation history
 
+### Nested Checklist Serialization
+
+Checklist items are stored as plain text in the note `content` field, one item per line. Child items use a 2-space indent prefix:
+
+```
+- [ ] Parent item
+  - [ ] Child item
+  - [x] Checked child item
+- [x] Another parent
+```
+
+- A line starting with `  - [ ]` or `  - [x]` (2 spaces) is parsed as a child of the preceding top-level item.
+- The `parentId` field on `ChecklistItem` is **runtime-only** — it is derived from position during parsing and never stored explicitly.
+- This format is fully backward compatible: flat checklists (no indented lines) parse identically to before.
+
 ### Collaboration Access Control
 
 Notes support sharing with other users on the same instance:

docs/FEATURES.md

@@ -37,6 +37,11 @@
 - Check/uncheck items with checkboxes
 - Checked items get strikethrough styling
 - Enter key adds new item, Backspace removes empty items
+- **Nested checklists**: single-level nesting for sub-tasks
+  - Tab / Shift+Tab to indent or outdent an item (keyboard)
+  - Drag handle horizontal swipe for mobile indentation
+  - Checking a parent item checks all its children
+  - Done section groups checked children under their parent label
 
 ### Image Attachments
 - Upload images via file picker or drag-and-drop