fix(app): restore question pager segments

[opencode-agent]

Summary

• use the V2 background token for inactive question pager segments so they remain visible in the current theme
• cover the four-question pager, its rendered segment color, and direct question navigation

Checks

• bun typecheck (packages/app)
• bun run typecheck:e2e (packages/app)
• bun typecheck (packages/session-ui)
• bun x prettier --check packages/app/e2e/regression/session-request-docks.spec.ts packages/session-ui/src/components/message-part.css
• Playwright regression attempted, but this shared host could not start Vite/esbuild because the OS process/thread limit was exhausted (EAGAIN)

Visual comparison

OC2 theme in light and dark modes, comparing dev (45cd8d7692) with this PR (3d3ebc99ce).

Requested by: @jayair (Jay via Slack)