feat(dashboard): GuruConnect v2 Support Codes view

Generate, list, and cancel attended-support codes (XXX-XXX-XXX), built
on the v2 codes API and existing UI primitives.

- Codes table: code in mono, status badge (pending+pulse/connected/
  completed/cancelled), bound client/machine, created-by, created
  (relative + absolute tooltip). Sticky header, skeleton load,
  actionable empty/error states.
- Generate opens a focused reveal modal showing the code large in
  JetBrains Mono with copy and a read-aloud instruction; the code is
  announced character-by-character for screen readers. Mint is ref-
  guarded so it creates exactly one code per open (no StrictMode dupe).
- Cancel via confirm dialog (POST /api/codes/:code/cancel), disabled for
  non-cancellable statuses; invalidates the codes query. List polls 7s.
- Shared API client now tolerates non-JSON 200 bodies, so the cancel
  endpoint's plain-text "Code cancelled" success no longer surfaces as a
  failure. Error-envelope handling unchanged.

Passed Code Review (no blockers after fixes) and local gates
(tsc/lint/build green).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

dashboard/src/components/layout/Sidebar.tsx

@@ -18,7 +18,7 @@ interface NavItem {
 const NAV: NavItem[] = [
   { to: "/machines", label: "Machines", Icon: MachinesIcon, enabled: true },
   { to: "/sessions", label: "Sessions", Icon: SessionsIcon, enabled: true },
-  { to: "/codes", label: "Codes", Icon: CodesIcon, enabled: false },
+  { to: "/codes", label: "Codes", Icon: CodesIcon, enabled: true },
   { to: "/users", label: "Users", Icon: UsersIcon, enabled: false },
 ];