fix(app): remove worktree user message accent - #45250

Summary

• remove the workspace-specific blue background and contrasting text override from sent user messages
• remove the now-unused workspace-session DOM attribute and virtualizer prop
• let worktree sessions use the same user-message styling as regular sessions in both color schemes

Testing

• bun test src --only-failures in packages/session-ui (112 passed)
• bun test --conditions=solid --only-failures --preload ./happydom.ts ./src/session/timeline in packages/app (19 passed)
• focused timeline performance unit coverage before and after the change (3 passed each)
• bun x prettier --check src/components/message-part.css in packages/session-ui
• git diff --check
• package typechecks are blocked by existing workspace dependency/type mismatches unrelated to this change

Requested by: @Brendonovich (Brendan via Slack)