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.

CHANGELOG.md

@@ -10,6 +10,13 @@ release PR may merge — and a section is closed at the commit that bumps it.
 
 ### Added
 
+- The assistant can now explain the **file viewer**, the **Tasks page**, your **Profile**
+  and the admin's **Users** page: ask it what a document's history button does, why a
+  `.tex` is shown instead of a PDF, how to stop a recurring job without losing it, what a
+  "cancelled" run means, what an encrypted account means when a password is forgotten, or
+  why it knows a member's age — and it answers from the in-app documentation instead of
+  guessing.
+
 - A **Files** section in the menu: everywhere you can reach, in one place — your home,
   your personal and the shared memory, the folders and projects shared with you, plus
   skills and documentation. Browse, open, download a folder as a ZIP, and upload, rename