fix(app): keep mobile question controls visible - #51771

Issue for this PR

Closes #51770

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Long questions and option lists can push Next and Submit off mobile screens. The height calculation falls back to the full viewport when there is no sticky timeline header, and the question text sits outside the scrollable area.

Measure the available session-panel height on mobile as well as desktop. Scroll the question text and options together, keeping the header and footer visible. Add a long-question story and mobile regression coverage.

How did you verify your code works?

• bun run check passed in the new worktree.
• Question component suite: 8 passed across Chromium and WebKit. Covers 393 × 852, 393 × 640, landscape, answer retention, submission, and desktop shortcuts.
• Agent-browser: captured before/after screenshots, scrolled to option 12, advanced, and submitted answers. Also checked the production build with a local fixture server, viewport resizing, and RTL.
• Production cold-session benchmark: 206.9 ms before, 214.2 ms after, one sample each. Two unrelated draft-entry assertions already failed on the baseline.

To reproduce locally: bun run test:components session-question-dock.spec.ts from packages/app, or open the LongQuestionRequest Storybook story at a mobile viewport.

Screenshots / recordings

Agent-browser captures of the production question component in the session story, at 393 × 852.

Before: Next is below the screen	After: content scrolls, Next stays visible

Long question scrolled to its options, with Back and Submit visible

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR