Closes #504.

Summary

• give the right-hand story workspace a true flex-1 min-h-0 content slot so Preview/Edit pages recalculate correctly after tab and route switches
• keep the prose editor save/status bar and the episode footer as reserved non-scrolling rows instead of letting them ride over the editor body
• add a Genesis opening-text regression test that switches Preview/Edit/Cuts repeatedly and asserts the save bar plus footer remain mounted separately from the editor body

Verification

• npm run typecheck
• npm run lint -- --quiet app/web/components/StoriesPage.tsx app/web/components/PreviewPanel.tsx app/web/components/PreviewPanel.test.tsx
• npm run app:build
• targeted Vitest remains blocked in this environment before collection by Unknown system error -122, write, including npx vitest run --coverage.enabled=false app/web/components/PreviewPanel.test.tsx

Visual / Viewport QA Notes

• the active right-panel view now lives inside a flex-1 min-h-0 wrapper in StoriesPage, so top workflow/nav strips and bottom publish/error strips reserve height instead of forcing the editor to overrun the column
• the plain prose editor shell in PreviewPanel is now overflow-hidden with a dedicated non-shrinking save/status row, and the bottom episode footer is also a non-shrinking sibling with its own background
• Genesis opening-text regression coverage switches Preview -> Edit -> Cuts -> Opening text -> Preview twice and asserts both the editor save bar and the footer remain present as separate chrome

Risks

• this changes the shared right-column flex stack for every story workspace, so any regression would most likely appear in other full-height story pages that depended on the old implicit h-full sizing
• the fix is layout-structural rather than route-specific, so follow-up visual review should still spot-check Whitepaper and future episode prose editors in addition to Genesis opening text