Move PRD/Spec headers out of cards and reposition action buttons

[MatheusBBarni]

Motivation

• Align the review workspace with the requested UI: show PRD/spec titles and action buttons outside the document cards, remove in-card header controls, and simplify the technical spec empty state.
• Ensure the Load button appears to the right of mode toggles for populated documents and that the PRD and Spec labels display the requested names.

Description

• Moved PRD and Spec headings and file paths out of the document cards and placed their action controls beside the titles in the review layout (src/components/MainWorkspace.tsx).
• Extracted header/action rendering out of the shared pane components by removing embedded eyebrow/title/action UI from DocumentPane and DocumentEmptyState and rendering children inside the cards instead (src/components/DocumentPane.tsx, src/components/DocumentEmptyState.tsx).
• Added a showModeButtons flag to DocumentActionBar to control whether the Preview/Edit toggles are shown and reordered the controls so Preview and Edit appear before Load (src/components/DocumentActionBar.tsx).
• Simplified the spec empty flow by removing in-card mode/load props and leaving only the prompt and a Generate Spec CTA within the empty card (src/components/SpecEmptyState.tsx).

Testing

• Built the frontend with bun run build, which ran tsc and produced a successful production bundle (build completed without errors).

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Codex Task