fix(core): fix ai input style in chat block and simply img rendering (#12943)

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

* **Style**
* Improved visual styling and cursor behavior for chat input send, stop,
and preference trigger buttons.
* Enhanced appearance and interactivity cues for the chat input
preference trigger.

* **Refactor**
* Simplified image preview grid by using CSS hover states for close
button visibility and switching to background images for previews.
* Streamlined image deletion process for a more intuitive user
experience.

* **Tests**
* Updated image upload test to wait for image container elements,
improving test reliability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/affine-cloud-copilot/e2e/utils/chat-panel-utils.ts

@@ -261,7 +261,7 @@ export class ChatPanelUtils {
   ) {
     await this.uploadImages(page, images);
 
-    await page.waitForSelector('ai-chat-input img');
+    await page.waitForSelector('ai-chat-input .image-container');
     await this.makeChat(page, text);
   }