Fix Orbit schema scroll when connect banner is open

What does this MR do?

Fixes the Orbit dashboard Schema tab being unscrollable while the "Connect to Orbit" banner is open, by giving the schema content a banner-independent scrollable height that mirrors the Explore tab's layout approach (.orbit-main-graph).

Because the schema graph panel no longer changes size when the banner toggles, this also fixes the schema graph rendering very small after the banner is closed (the SVG viewBox was fitted to the squeezed panel and never re-fitted).

Why was this MR needed?

On desktop viewports (>= 992px), the Schema tab content was a pure flex-fill inside the fixed-height .orbit-app container and was clipped by overflow: hidden. Opening the Connect banner squeezed the tab content instead of letting the page scroll, cutting off the schema with no way to reach it. The Explore tab already handles this correctly by giving its map an explicit height independent of the banner, so overflow reaches the page scroll container.

Verification

• Reproduced the bug on a clean master checkout at 1440x900: schema row squeezed to 161px with overflow: hidden, graph clipped 310px below the viewport with no scrollbar; after closing the banner the panel grew but the SVG viewBox stayed stale.
• With this fix: schema row holds a constant banner-independent height, the page scrolls while the banner is open, and the panel size (and therefore the viewBox fit) is unchanged by banner toggling. Explore tab behavior unchanged.
• yarn jest ee/spec/frontend/orbit: 442/442 pass. ESLint and Stylelint clean on the touched files.

What are the relevant issue numbers?

Closes gitlab-org/orbit/knowledge-graph#890 (closed)

Does this MR meet the acceptance criteria?

• Verified in the browser (before/after) on a local GDK EE instance
• Existing tests passing; CSS-only layout change with no new tests
• Follows project style guide
• No breaking changes introduced