fix(app): restore worktree message accent for further trial - #45486

Summary

• Revert fix(app): remove worktree user message accent #45250 to restore blue user-message bubbles in worktree sessions for another trial, not as a permanent design decision.
• Restore the workspace-session virtualizer prop, DOM attribute, and existing accent/contrast CSS tokens. Regular sessions remain unchanged.
• Preserve all intervening timeline changes; no feature flag or unrelated refactor.

Validation

• Session UI unit tests: 122 passed.
• App timeline unit tests: 22 passed.
• bun typecheck passed in both packages/app and packages/session-ui.
• Prettier checks on all three changed files and git diff --check passed.
• Production-build Chromium comparison: light/dark × regular/worktree, before and after (8 states). Worktree bubbles restore #3b5cf6 with white text; regular bubble crops have zero changed pixels. Bubble geometry is unchanged.
• Real app with mocked session/project/message API data, seeded tabs, 1000×720 viewport, scale 1, English locale, reduced motion. No recreated UI.
• Production cached-tab-switch benchmark passed before and after: first-correct observation 31.2 → 31.5 ms; stable observation 120.7 → 121.8 ms; zero blank or wrong-destination samples. One sample each, not a statistical performance claim.
• Before: ff5b5d00f98307f1ef85256f76a665b8ca0f8753; after: 2efe6cf71eecd1188a50014cd5e09702fa5a1b42.

Requested by: @Brendonovich (Brendan via Slack)