fix(core): should not be able to comment with empty content (#13061)

fix AF-2712

#### PR Dependency Tree


* **PR #13061** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* The comment editor now disables the commit button when the editor is
empty, preventing accidental submissions.
* The commit action is now triggered by pressing Enter together with
CMD/CTRL, instead of Enter without Shift.

* **Style**
* The disabled state styling for the commit button now matches the
native HTML `disabled` attribute for improved consistency.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/modules/comment/services/snapshot-helper.ts

@@ -5,6 +5,7 @@ import {
   MarkdownAdapter,
 } from '@blocksuite/affine/shared/adapters';
 import {
+  type BlockModel,
   type DocSnapshot,
   nanoid,
   type Store,
@@ -152,4 +153,31 @@ export class SnapshotHelper extends Service {
       return undefined;
     }
   }
+
+  isDocEmpty(store?: Store): boolean {
+    if (!store) {
+      return true;
+    }
+
+    const checkBlock = (block: BlockModel) => {
+      if (block.text && block.text.length > 0) {
+        return false;
+      }
+      const children = block.children;
+      for (const child of children) {
+        if (!checkBlock(child)) {
+          return false;
+        }
+      }
+      return true;
+    };
+
+    const blocks = store.blocks.peek();
+    for (const block of Object.values(blocks)) {
+      if (!checkBlock(block.model)) {
+        return false;
+      }
+    }
+    return true;
+  }
 }