Add reminders and a notification inbox

## Goal

A mention only helps if the recipient notices it. Add a notification inbox for mentions and comment replies, and let users set reminders on documents.

## Tasks

- [ ] Add a notification inbox to the workspace shell with unread state, timestamps, and links back to the relevant document or conversation.
- [ ] Create notifications for document mentions, comment replies, and direct mentions in comments.
- [ ] Let a user set, change, or remove a reminder on a page or collection row.
- [ ] Deliver due reminders to the same inbox and take the user to the document when opened.
- [ ] Add read, unread, and mark-all-read actions, and prevent notifications from revealing content the user can no longer access.

## Acceptance criteria

- A mention or comment reply appears in the right user's notification inbox.
- A user can set a reminder on a page or row and receive it at the selected time.
- Opening a notification takes the user to its source when that source is still accessible.
- Read and unread state persists between sessions.
- Losing access to a document prevents its title or content from leaking through notifications.

## Out of scope

- Email, mobile push, and other delivery channels outside the Cortext workspace.

Start by locating the workspace shell, mention and comment-reply handling, and the page or collection-row entry points for reminders. Trace how access checks and persisted user state work before deciding where inbox and reminder behavior belongs. Done means accessible notifications and reminders appear with persistent read state, navigation, and no leaked document content.

Written by the indexing model from the issue text.