feat: checklist improvements — drag reorder and hide completed

- Add drag-to-reorder via svelte-dnd-action with grip handles
- Add "hide completed" toggle with collapsible done section
- Each checklist item gets a unique ID for stable drag operations
- E2E test for hide/show completed items

Closes #11

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

pnpm-lock.yaml

@@ -61,6 +61,9 @@ importers:
       markdown-it:
         specifier: ^14.1.1
         version: 14.1.1
+      svelte-dnd-action:
+        specifier: ^0.9.69
+        version: 0.9.69(svelte@5.53.7)
       tiptap-markdown:
         specifier: ^0.9.0
         version: 0.9.0(@tiptap/core@3.20.1(@tiptap/pm@3.20.1))
@@ -2900,6 +2903,11 @@ packages:
       svelte: ^4.0.0 || ^5.0.0-next.0
       typescript: '>=5.0.0'
 
+  svelte-dnd-action@0.9.69:
+    resolution: {integrity: sha512-NAmSOH7htJoYraTQvr+q5whlIuVoq88vEuHr4NcFgscDRUxfWPPxgie2OoxepBCQCikrXZV4pqV86aun60wVyw==}
+    peerDependencies:
+      svelte: '>=3.23.0 || ^5.0.0-next.0'
+
   svelte@5.53.7:
     resolution: {integrity: sha512-uxck1KI7JWtlfP3H6HOWi/94soAl23jsGJkBzN2BAWcQng0+lTrRNhxActFqORgnO9BHVd1hKJhG+ljRuIUWfQ==}
     engines: {node: '>=18'}
@@ -6037,6 +6045,10 @@ snapshots:
     transitivePeerDependencies:
       - picomatch
 
+  svelte-dnd-action@0.9.69(svelte@5.53.7):
+    dependencies:
+      svelte: 5.53.7
+
   svelte@5.53.7:
     dependencies:
       '@jridgewell/remapping': 2.3.5