style(webview): chat polish batch

Extracted from #39. A batch of small, independent visual fixes, intended as one PR rather than five:

- CopyButton gains an opaque/blurred background so it stops visually colliding with the text beneath it
- FileReferenceCard becomes an outlined card using editor/panel theme tokens
- SessionList heading renamed from "Chat history" to "Sessions"
- UserMessage bubble and timestamp layout tweaks; assistant timestamp pinned in a reserved bottom strip with the copy button repositioned

**Size**: S | **Depends on**: nothing | **Tier**: 0 (quick win)

Look for the CopyButton, FileReferenceCard, SessionList, and UserMessage components in the webview code. Each change is a small visual tweak: adjust background styles, apply theme tokens, rename a heading, and reposition timestamps and buttons. Test by running the extension in a VS Code development host and checking the chat interface.

Escrita pelo modelo de indexação a partir do texto da issue.