docs: add remove cover page design

docs/superpowers/specs/2026-06-16-remove-cover-page-design.md

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+# Remove Cover Page Design
+
+## Goal
+
+Remove the cover page feature completely from the teaching book workflow, including UI entry points, editable cover state, and legacy saved `cover` data in book JSON.
+
+## Scope
+
+In scope:
+
+- Remove `TeachingBook.cover` from the domain model.
+- Change `TeachingBook.selectedId` from `'cover' | DesignId` to `DesignId | null`.
+- Make newly created books start with no selected page.
+- Migrate existing stored book JSON by deleting `cover`.
+- Migrate existing `selectedId: 'cover'` to the first lesson id, or `null` when the book has no lessons.
+- Remove the sidebar "封面" navigation item.
+- Remove cover rendering and cover update events from the A4 workspace.
+- Stop passing cover state through `WorkspaceView.vue`.
+- Delete the unused `CoverPage.vue` component and cover-specific CSS.
+- Update tests for the domain model, store behavior, sidebar, workspace, DB migration, and route persistence.
+
+Out of scope:
+
+- Removing book names from the book list. `BookRecord.name` remains the management/display name for each saved book.
+- Changing lesson rendering, print layout, Markdown export, ZIP export, or generated lesson content.
+- Creating a user-facing migration screen.
+
+## Data Model
+
+`TeachingBook` becomes lesson-only:
+
+```ts
+export interface TeachingBook {
+  schemaVersion: typeof BOOK_SCHEMA_VERSION
+  designs: TeachingDesign[]
+  selectedId: DesignId | null
+  updatedAt: string
+}
+```
+
+`createEmptyBook()` returns an empty `designs` array and `selectedId: null`.
+
+The schema version can stay at `1` because the server migration normalizes stored JSON at load/open time. The app does not need to preserve old cover data.
+
+## Legacy Data Migration
+
+The server will normalize every stored book when opening the SQLite database:
+
+- Parse `books.data`.
+- Delete `data.cover` when present.
+- If `data.selectedId === 'cover'`, set it to `data.designs[0].id` when a first lesson exists, otherwise `null`.
+- If `data.selectedId` points to a missing lesson id, set it to the first lesson id or `null`.
+- Persist the normalized JSON back into the row only when it changed.
+
+This ensures old database rows no longer contain `cover` after the app starts, not just after a user edits the book.
+
+## UI Behavior
+
+The workspace has no cover page:
+
+- `LessonSidebar.vue` lists lessons only.
+- Empty books continue to show the upload dropzone instead of an A4 page.
+- `A4Workspace.vue` renders `TeachingDesignPage` only when there is a selected lesson.
+- `WorkspaceView.vue` no longer passes cover props or listens for cover update events.
+
+Selection behavior becomes:
+
+- Importing lessons into an empty book selects the first imported/sorted lesson.
+- Generating a lesson selects the generated lesson.
+- Deleting the selected lesson selects the next lesson, then previous lesson, then `null`.
+- Clearing all lessons sets `selectedId` to `null`.
+
+## Removed Code
+
+Delete:
+
+- `src/components/CoverPage.vue`
+- Cover-specific CSS rules in `src/style.css`
+
+Keep:
+
+- Book list names and backend book metadata.
+- Existing print/export behavior, which already renders lessons without a cover page.
+
+## Testing
+
+Domain tests verify:
+
+- New books do not have `cover`.
+- New books start with `selectedId: null`.
+- Independent book creation still produces independent lesson arrays.
+
+DB tests verify:
+
+- Opening a database migrates old `cover` data out of saved book JSON.
+- Old `selectedId: 'cover'` becomes the first lesson id when lessons exist.
+- Old `selectedId: 'cover'` becomes `null` when no lessons exist.
+- Invalid selected ids are normalized.
+
+Store/component tests verify:
+
+- `useTeachingBook` no longer exposes `updateCover`.
+- Clear/delete/import/generate selection behavior uses `null` or lesson ids only.
+- `LessonSidebar` does not render a cover button.
+- `A4Workspace` does not import or render `CoverPage`.
+- `WorkspaceView` does not pass cover props.
+
+Verification will run the frontend tests, backend tests, and production build.