docs: v1 docs refresh (batch 8) (#1797)

* docs(rbac): refresh Users panel screenshots to current build

Recapture users-list, add-user-form, role-selector, and delete-confirm
against the live app so they reflect the current Community RBAC parity
UI. Update alt text to match.

* docs(sso): refresh SSO settings screenshots and document last-provider lockout guard

Screenshots were captured against a stale build; refresh with current UI.
Also documents the guard that blocks disabling or removing the last
enabled SSO provider while SSO-only mode is active, and corrects the
Recovery settings path (Operations, not System).

* docs(sso): refresh SSO quickstart screenshots to current build

Screenshots were captured against a stale build; recapture the provider
overview and the LDAP test-connection failure state against current UI.
Page text already matched the implementation, so no content changes.

* docs(alerts): refresh Alerts and Notifications for current RBAC and retention behavior

Verified every claim against current backend/frontend code and the live app.
Corrected two factual errors: stack alert rule add/delete visibility now
follows the stack:edit permission (roles renamed since the last refresh,
not an admin-only gate), and the notification_history cap is a 500-row
per-stack / 1000-row per-node-unattached pair enforced on the monitor's
30-second cleanup pass, not a flat 100-row per-node cap evicted on insert.

Added the stack_taken_down category (also part of the built-in bell
quieting rule, previously undocumented) and the seven history-only
categories that are filterable but never dispatched externally. Noted the
two vulnerability-scan retention controls now sharing the Data Retention
page, with a cross-link to their own doc.

Recaptured all eight screenshots against the production node.

* docs(fleet-sync): remove stale Admiral gate claim, refresh replica screenshots

Fleet Sync's replication, anchor, and demote mechanics have been free on
Community since the paid gates were dropped from sync-status and
reset-anchor; the Prerequisites table still claimed the control instance
needed Admiral. Also documents the one-click "Reset anchor on peer"
recovery banner on Settings -> Nodes, verified live against a real
control/replica pair, and replaces all four screenshots with current
captures.

docs/features/rbac.mdx

@@ -62,7 +62,7 @@ Both plans use the same five built-in roles and the same additive stack and node
 The Users panel lives at **Settings · Users**, under the **Access** group of the settings sidebar. It is visible only to users with the Admin role, and is hidden when a remote node is the active selection.
 
 <Frame>
-  <img src="/images/rbac/users-list.png" alt="Settings Users panel showing the Access sidebar selection, a SCOPE global chip, an OPERATORS 2 counter, an Add user button, and a four-column table (Username, Role, Created, Actions) with two rows: admin marked (you) with a disabled trash icon and viewer with active edit and trash icons." />
+  <img src="/images/rbac/users-list.png" alt="Settings Users panel showing the Access sidebar selection, a SCOPE global chip, an OPERATORS 3 counter, an Add user button, and a four-column table (Username, Role, Created, Actions) with three rows: an Admin row marked (you) with a disabled trash icon, a second Admin row, and a Viewer row, both with active edit and trash icons." />
 </Frame>
 
 The table shows one row per user with their **Username**, **Role** badge, account **Created** date, and per-row action icons. The signed-in admin's row carries a small `(you)` marker after the username and the delete icon is disabled, so you cannot lock yourself out by deleting your own account.
@@ -157,7 +157,7 @@ If you need to track repeated lockouts, the audit log records each failed login
 Click the trash icon on a user row. A confirmation modal with the kicker `USERS · DELETE · IRREVERSIBLE` opens.
 
 <Frame>
-  <img src="/images/rbac/delete-confirm.png" alt="Delete confirmation modal with the kicker USERS DELETE IRREVERSIBLE, the title Delete user viewer in italic display type, the body Removes the user immediately. They lose access right away., and Cancel and Delete buttons on the lower right." />
+  <img src="/images/rbac/delete-confirm.png" alt="Delete confirmation modal with the kicker USERS DELETE IRREVERSIBLE, the bold title Delete user site-viewer, the body Removes the user immediately. They lose access right away., and Cancel and Delete buttons on the lower right." />
 </Frame>
 
 The body reads: