Trim the Servers page down to a server list + add-server flow

Scheduled-task browsing/editing already lives on each server's detail
page (ServerDetail.tsx), so the overview page's filters, task-group
listing, and add/edit-task form were pure duplication -- the only
things it needs to do are list servers (as clickable cards) and let
an admin register new ones / issue or rotate agent tokens.

Renamed ServersTasks.tsx -> Servers.tsx to match its new, narrower
scope; updated the nav label, route, and ServerDetail's back-link
text from "Servers & Tasks" to "Servers" accordingly. No server-side
or API changes -- this only removes now-redundant frontend code.

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

web/src/layout/AppShell.tsx

@@ -29,7 +29,7 @@ interface NavItem {
 
 const NAV_ITEMS: NavItem[] = [
   { to: "/", label: "Dashboard", icon: <IconLayoutDashboard size={20} /> },
-  { to: "/servers", label: "Servers & Tasks", icon: <IconServer2 size={20} /> },
+  { to: "/servers", label: "Servers", icon: <IconServer2 size={20} /> },
   { to: "/dns", label: "DNS", icon: <IconWorld size={20} /> },
   { to: "/ipam", label: "IP Addresses", icon: <IconNetwork size={20} /> },
   { to: "/secrets", label: "Secrets", icon: <IconKey size={20} /> },