[Web/PWA] Add manual and confirmed AI memory saving

[AlphaMao1]

Parent

#11

What to build

Add intentional memory capture for the reader assistant. Users can save memory with /记住, and the assistant may suggest a memory that the user confirms before saving. Saved memories should sync and influence later answers without fully automatic extraction.

Acceptance criteria

• /记住 creates a memory only after explicit user action.
• AI-suggested memories require user confirmation before persistence.
• Memories support user profile, book gist, and concept categories or equivalent stable categories.
• Saved memories sync across signed-in browser/device sessions.
• Relevant memories are injected into later AI prompts in a bounded way.
• Deleting a book does not automatically delete cross-book memories.
• There is no fully automatic memory extraction path in the first version.

Blocked by

• [Web/PWA] Add AI chat sidebar and bottom sheet with reading context routing #18