feat(settings): reorganize the settings hub into domain groups (#1321)

* refactor(settings): split System Limits and regroup the hub

System Limits had grown into a grab-bag of host alert thresholds, Docker
cleanup, and mesh data-plane controls under one mislabeled section. Split it
into Host Alerts, Docker & Storage, and Fleet Mesh, and split Developer into
Developer Diagnostics and Data Retention. Reorganize the sidebar into ten
domain groups: Personal, Access, Infrastructure, Monitoring, Notifications,
Automation, Organization, Security, Operations, Help.

Each section now saves only its own keys, so a concurrent edit in one section
no longer clobbers another. Data Retention sends the audit-log window only on
a paid plan, matching the field's existing visibility, so a Community save no
longer fails on a key the operator cannot set. NumberChip moves to a shared
module and the toggle reuses the existing shared component. The /settings API
is unchanged.

* test(settings): cover registry structure and per-section save payloads

Add structural invariants for the ten-group registry (every item maps to a
real group, ids are unique, the System Limits and Developer splits land in the
right groups with the right gates, renamed labels and the Registries paid gate
hold) and per-section payload tests asserting each split section patches only
its own keys, including the Community path where Data Retention omits the paid
audit-log key.

* docs(settings): document the regrouped settings hub

Rewrite the settings reference for the ten-group layout, replace the System
Limits page with Host Alerts, Docker & Storage, and Fleet Mesh, and document
the prune-on-update, reclaimable-space banner, and mesh auto-recreate settings
that were previously undocumented. Update the Settings navigation breadcrumbs
across the feature docs and refresh the affected screenshots.

* fix(settings): show Access sections as instance-global, not operator-scoped

License, Users, SSO, and API Tokens are instance-global settings but the
masthead scope label rendered them as operator-scoped because it keyed off the
old Identity group. Only Personal sections (account, appearance) are
operator/browser-scoped now; everything else reads as global.

Also add a compile-time exhaustiveness guard to the section switch so a future
SectionId added without a matching case fails the build instead of silently
rendering a blank panel.

* docs(settings): remap remaining settings breadcrumbs to the new groups

Update the navigation breadcrumbs that still pointed at the removed Identity,
Alerts, and Advanced groups: API Tokens and Users now sit under Access, Webhooks
under Automation, Labels under Organization, App Store under Infrastructure,
Appearance under Personal, and scan policies under Security > Vulnerability
Scanning. Correct the settings reference scope note so Access reads as global.

* docs(settings): remap renamed-section breadcrumbs across feature docs

Sweep every feature, operations, getting-started, and reference page for
navigation paths that still named the renamed settings sections, and point them
at the current ones: Security becomes Security > Vulnerability Scanning,
Notifications becomes Notifications > Channels, Routing becomes Notifications >
Notification Routing, and Developer becomes Operations > Developer Diagnostics
(with its retention windows under Operations > Data Retention). App Store moves
under Infrastructure and the four-group overview in the getting-started intro is
rewritten to the ten groups. Separators each page already used are preserved.

docs/features/rbac.mdx

@@ -58,10 +58,10 @@ Community accounts use the **Admin** and **Viewer** roles. Admiral adds the inte
 
 ## Managing users
 
-The Users panel lives at **Settings · Users**, under the **Identity** 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.
+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 Identity sidebar selection, a SCOPE operator 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 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." />
 </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.