Convert public directory from submodule to normal directory

- Remove public from git submodule tracking
- Add all public directory contents as regular files
- Fixes GitHub Actions error: "No url found for submodule path 'public' in .gitmodules"

🤖 Generated with [Claude Code](https://claude.ai/code)

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

public/book/1/AGlkOnBsYzo0aHFqZm43bTZuNWhubzNkb2FtdWhnZWY/card/book_0.json

@@ -0,0 +1,102 @@
+[
+  {
+    "send": 1,
+    "card": 39
+  },
+  {
+    "send": 2,
+    "card": 39
+  },
+  {
+    "send": 3,
+    "card": 40
+  },
+  {
+    "send": 4,
+    "card": 39
+  },
+  {
+    "send": 5,
+    "card": 40
+  },
+  {
+    "send": 6,
+    "card": 40
+  },
+  {
+    "send": 7,
+    "card": 40
+  },
+  {
+    "send": 8,
+    "card": 39
+  },
+  {
+    "send": 9,
+    "card": 40
+  },
+  {
+    "send": 10,
+    "card": 39
+  },
+  {
+    "send": 11,
+    "card": 39
+  },
+  {
+    "send": 12,
+    "card": 40
+  },
+  {
+    "send": 13,
+    "card": 40
+  },
+  {
+    "send": 14,
+    "card": 40
+  },
+  {
+    "send": 15,
+    "card": 40
+  },
+  {
+    "send": 16,
+    "card": 40
+  },
+  {
+    "send": 17,
+    "card": 40
+  },
+  {
+    "send": 18,
+    "card": 40
+  },
+  {
+    "send": 19,
+    "card": 27
+  },
+  {
+    "send": 20,
+    "card": 40
+  },
+  {
+    "send": 21,
+    "card": 40
+  },
+  {
+    "send": 22,
+    "card": 40
+  },
+  {
+    "send": 23,
+    "card": 40
+  },
+  {
+    "send": 24,
+    "card": 40
+  },
+  {
+    "send": 25,
+    "card": 40
+  }
+]