Fix missing flask width/height sliders in production workflow build

[Testtesttestestes]

Motivation

• The control sliders Flask Width / Flask Height were visible during npm run dev but became inaccessible in the production bundle due to the control panel being centered with translate-based positioning and lacking a reliable max-height/scroll setup.
• The CI workflow only validated presence of the Gravity label in the built bundle, so regressions for other sliders were not detected.
• Skills used: none (no repository skills required for this change).

Description

• Reworked the control panel wrapper in src/App.tsx to use a full-width flex container and removed left-1/2 -translate-x-1/2 centering so the panel can reliably center and scroll across viewports.
• Made the panel pointer-events-auto while the outer wrapper uses pointer-events-none so the panel remains interactive and does not get clipped by transforms, and set an explicit style={{ maxHeight: 'calc(100dvh - 3rem)' }} to enable vertical scrolling for deep viewports.
• Updated .github/workflows/build-static-page.yml to rename the verification step and to grep for Flask Width and Flask Height in addition to Gravity to catch similar regressions in CI.

Testing

• Ran npm run lint (TypeScript type-check) and it succeeded.
• Ran npm run build and the production build completed successfully.
• Served the produced dist with python3 serve_dist.py and captured a Playwright screenshot to confirm the sliders are present and reachable in the built output (visual check succeeded).

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Codex Task