headscale is core, so stop offering to install it

Reported as: the routes are unreachable and there is no dock tile, on a server where
officer-headscale is up and healthy. Both symptoms, one cause.

Availability is derived ONLY from the sidecar_installs table — `usable` is the rows
with status='installed' AND enabled, and every capability mapped to a sidecar outside
that set is added to `unavailable`. A CORE sidecar never gets a row there, because
core processes are started by pm2 from the generated ecosystem file and never go
through the app store. So `headscale` and `vpn` were permanently unavailable, which
withheld the dock manifest AND put /headscale into deniedRoutes for the route guard.

The design already knew. catalogue.test.ts has a test called "does not offer to
install the baseline", and it has been FAILING since headscale was promoted:

  Expected to not contain: "officer-headscale"

docs/secret-store.md predicted it in as many words — "moving headscale into the light
profile also removes it from the app store automatically: catalogue.test.ts asserts
the catalogue equals full − light, so the test fails until the entry is deleted". The
entry was never deleted, and the failing test was never read.

So: entry removed, and the tile moved to CORE_DOCK_ITEMS, where the other things that
are always present live. DashboardLayout filters every tile through canVisit(), so a
member still never sees it — the capability is kind: 'admin'.

The entry's existingFields (URL + API key) are not lost. Servers are added from the
Servers view inside the app — ServersView.tsx, ServerForm.tsx, useHeadscaleServers.ts
— which is where they were really configured; the app-store form was a second place to
type the same two values.

Verified: catalogue.test.ts 19 pass/1 fail → 20 pass/0 fail, tsgo clean.

PRE-EXISTING, not touched: 10 other tests fail on master, 8 of them in
src/servers/capabilities. Confirmed identical before and after this change by
stashing it and re-running. Worth a look but not this change's business.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/apps/officer-web/Screens/Dashboard/Layout/Dock.tsx

@@ -174,6 +174,10 @@ export const CORE_DOCK_ITEMS: DockItem[] = [
   { label: 'Monitor', to: '/system-monitor', icon: Activity, color: '#0ea5e9' },
   { label: 'Activity', to: '/activity', icon: Radio, color: '#f59e0b' },
   { label: 'Dashboards', to: '/dashboards', icon: LayoutGrid, color: '#8b5cf6' },
+  // Core because the tailnet is the perimeter — origin checking was removed on the grounds that the
+  // tailnet stands in its place, so administering it cannot be an optional extra. It is `kind: 'admin'`,
+  // and DashboardLayout filters every tile through canVisit(), so a member never sees this one.
+  { label: 'Headscale', to: '/headscale', icon: Network, color: '#818cf8' },
   // Core by necessity: the store is how every other feature arrives, so it can never be one of the
   // things that disappears when uninstalled.
   { label: 'App store', to: '/app-store', icon: Store, color: '#64748b' },