✨ feat(ui): add clear conversation button to input area with symmetric layout

- Add clear context button positioned on the left side of input area
- Create symmetric layout with clear button (left) and send button (right)
- Standardize both buttons to 32x32px size for consistent appearance
- Apply distinct styling: gray background for clear (red on hover), purple for send
- Add smooth transition animations for better user experience
- Align buttons vertically centered for improved visual balance

The clear conversation button provides quick access to context clearing
functionality directly from the input area, matching the design patterns
shown in Semi Design documentation and improving overall user workflow.

web/src/index.css

@@ -307,4 +307,8 @@ code {
 
 .semi-chat-chatBox-action {
   column-gap: 0 !important;
+}
+
+.semi-chat-inputBox-clearButton.semi-button .semi-icon {
+  font-size: 20px !important;
 }