Add comments to pages and rows

## Goal

People need somewhere to discuss a document without turning its body into a comment thread.

Add document-level comments to pages and collection rows. Every comment should show who wrote it and when, and follow the document's access rules.

## Tasks

- [ ] Add comments to pages and item detail views, with a visible count and an obvious way to open the conversation.
- [ ] Let users add comments, edit or delete their own comments, and resolve or reopen a conversation.
- [ ] Show the author and timestamp for each comment and keep the thread ordered as new replies arrive.
- [ ] Respect document permissions when reading or writing comments and stop comments from appearing on content the current user cannot access.
- [ ] Send new replies and user mentions to the notification inbox instead of storing the conversation in the document body.

## Acceptance criteria

- A user with access can start and reply to a conversation on a page or row.
- The comment count and thread update after a new reply.
- A user can edit or delete their own comment and resolve or reopen the conversation.
- Someone without access to the document cannot read or add its comments.
- A new reply or mention produces a notification for the relevant user.

## Out of scope

- Anchoring comments to a text selection or individual block. The first version keeps the conversation at document level.

No files, tests, or entry points are named. Start by locating the existing page and collection-row detail views, document permission checks, comment-related UI or data models, and the notification inbox; define the work as complete when the listed acceptance criteria pass for both pages and rows.

Written by the indexing model from the issue text.