[Web/PWA] Add Markdown export and deletion semantics

[AlphaMao1]

Parent

#11

What to build

Finish the first-version data lifecycle: users can export reading material as Markdown, delete books to reclaim storage, and trust that sync will not resurrect deleted rows. Direct Obsidian writing is out of scope; Markdown download is the supported bridge.

Acceptance criteria

• A user can download Markdown containing book metadata, highlights, notes, and useful AI/memory references where appropriate.
• The UI does not claim direct Obsidian vault writing.
• Deleting a book removes the Supabase Storage EPUB object.
• Deleting a book removes or invalidates local cached EPUB data.
• Deleting a book removes RAG chunks/vectors/index records for that book.
• Book-linked annotations and conversations are soft-deleted with deleted_at and hidden after sync.
• Cross-book memories are retained unless the user explicitly confirms memory deletion.
• A two-device smoke test proves deleted data is not resurrected.

Blocked by

• [Web/PWA] Add selection floating menu for highlight, note, and Ask AI #16
• [Web/PWA] Add client-side EPUB indexing with resumable RAG jobs #17
• [Web/PWA] Add manual and confirmed AI memory saving #19
• [Web/PWA] Add annotations notepad and reading digest review surfaces #26