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/files.md

@@ -24,7 +24,7 @@ A place they have no access to simply is not in the list — there is nothing to
 
 Clicking a place opens a file explorer: one folder at a time, folders first, with size and dates. From there:
 
-- **Clicking a file opens it** in the usual file viewer — Markdown rendered, images, PDFs, colored code, plain text.
+- **Clicking a file opens it** in the usual file viewer — Markdown rendered, images, PDFs, colored code, plain text. What it does with each kind, and where a Markdown file can be edited, is [file-viewer.md](file-viewer.md).
 - **Clicking a folder goes into it**; the breadcrumb at the top walks back out, and the browser's back button works too. The address bar carries the folder, so a user can bookmark or paste a link to exactly where they are.
 - **The listing is live.** A file you create from a conversation, or another member uploads, appears within a second without a refresh. If a user says "it's not there", ask them to check the folder rather than assuming the write failed — but the page updates on its own, so a truly missing file is missing.
 - **ZIP download** — the button in the toolbar downloads the whole open folder as a single archive. Useful when someone wants "all the photos" rather than one file.
@@ -52,6 +52,7 @@ The two memory stores appear as ordinary folders, and this is the first place a
 
 ## Related
 
+- [file-viewer.md](file-viewer.md) — the page a file opens into: what is rendered, what is editable, and the version history
 - [shared-folders.md](shared-folders.md) — who may see a shared folder, and read vs write access
 - [projects.md](projects.md) — projects have their own explorer on the project page, the same one this page uses
 - [memory.md](memory.md) — what goes in each memory store, and why shared memory asks for confirmation