Replace the previous monolithic MainContent.jsx with a typed one and
extract focused subcomponents/hooks to improve readability, local state ownership,
and maintainability while keeping runtime behavior unchanged.
Key changes:
- Replace `src/components/MainContent.jsx` with `src/components/MainContent.tsx`.
- Add typed contracts for main-content domain in `src/components/main-content/types.ts`.
- Extract header composition into:
- `MainContentHeader.tsx`
- `MainContentTitle.tsx`
- `MainContentTabSwitcher.tsx`
- `MobileMenuButton.tsx`
- Extract loading/empty project views into `MainContentStateView.tsx`.
- Extract editor presentation into `EditorSidebar.tsx`.
- Move editor file-open + resize behavior into `useEditorSidebar.ts`.
- Move mobile menu touch/click suppression logic into `useMobileMenuHandlers.ts`.
- Extract TaskMaster-specific concerns into `TaskMasterPanel.tsx`:
- task detail modal state
- PRD editor modal state
- PRD list loading/refresh
- PRD save notification lifecycle
Behavior/compatibility notes:
- Preserve existing tab behavior, session passthrough props, and Chat/Git/File flows.
- Keep interop with existing JS components via boundary `as any` casts where needed.
- No intentional functional changes; this commit is structural/type-oriented refactor.
Validation:
- `npm run typecheck` passes.
- `npm run build` passes (existing unrelated CSS minify warnings remain).
