docs: the file viewer, the Tasks page, profiles and user administration

Four gaps off the coverage map, written for the in-app assistant:

- file-viewer.md — what each kind renders to, the live reload, editing a
  Markdown file and the conflict banner, git history mode, and why a `.tex`
  must be shown instead of a PDF built from it.
- tasks-page.md — the four sections, disable-vs-delete, where each kind's
  result lands, and that there is no "new task" button because tasks are
  created in conversation.
- profile.md — display name, language, password, and what an encrypted
  account means when the password is forgotten.
- users.md — creating a member and the irreversible encryption choice, the
  directory profile that feeds the agents' prompt, deactivating vs deleting,
  and the per-person event-triage interval.

Indexed in docs/index.md, cross-linked from files.md, tasks.md and access.md.

docs/tasks.md

@@ -46,4 +46,4 @@ Common questions and the honest answers:
 - *"It has been going for ages."* — a task has no time limit; stopping it with ■ is always available, and stopping is not the same as failing.
 - *"Where did the result go?"* — into this conversation, always. If it is not there yet, the task has not finished.
 - *"It's stuck."* — check whether it is asking for something: an approval or a question waiting at the top of the strip, or in the Inbox if the card was closed earlier.
-- *"Show me everything that ever ran."* — the **Tasks** page (sidebar → Tasks) has the full history, including scheduled jobs; the strip only covers the current conversation.
+- *"Show me everything that ever ran."* — the **Tasks** page (sidebar → Tasks) has the full history, including scheduled jobs; the strip only covers the current conversation. See [tasks-page.md](tasks-page.md).