fix(core): sidebar ai layout (#13215)

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

## Summary by CodeRabbit

* **Style**
* Improved chat panel layout with flexible vertical sizing and
alignment.
* Updated padding for chat panel titles to ensure consistent appearance
even if CSS variables are missing.

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

packages/frontend/core/src/blocksuite/ai/chat-panel/ai-title.ts

@@ -30,7 +30,7 @@ export class AIChatPanelTitle extends SignalWatcher(
     .ai-chat-panel-title {
       background: var(--affine-background-primary-color);
       position: relative;
-      padding: 8px var(--h-padding);
+      padding: 8px var(--h-padding, 16px);
       width: 100%;
       height: 36px;
       display: flex;

packages/frontend/core/src/blocksuite/ai/chat-panel/index.ts

@@ -40,6 +40,13 @@ export class ChatPanel extends SignalWatcher(
 
       .chat-panel-container {
         height: 100%;
+        display: flex;
+        flex-direction: column;
+      }
+
+      ai-chat-content {
+        height: 0;
+        flex-grow: 1;
       }
 
       .chat-loading-container {