Make the empty thread panel fill BaseCard (#8690)

Use border-box to make maintaining the layout more intuitive.

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

res/css/views/right_panel/_ThreadPanel.scss

@@ -316,6 +316,8 @@ limitations under the License.
     bottom: 0;
     left: 0;
     padding: 20px;
+    box-sizing: border-box; // Include padding and border
+    width: 100%;
 
     h2 {
         color: $primary-content;