feat(dashboard): GuruConnect v2 Sessions view (pass 2)

Active-sessions table with consent-state badges, viewer-token Join,
and disconnect, built on the v2 session API and existing UI primitives.

- Sessions table: machine, mode (managed/attended), consent badge
  (granted/pending+pulse/denied/not_required), viewers, started,
  duration, status. Sticky header, skeleton load, empty/error states.
- Join action mints a session-scoped viewer token
  (POST /api/sessions/:id/viewer-token) and reveals it with the
  /ws/viewer relay URL and copy buttons. The static viewer.html is
  intentionally not targeted: it sends the raw login JWT, which the v2
  viewer plane rejects. In-dashboard web viewer ships in a later pass.
- Authz split mirrors the server mint gate: admin or control permission
  gets Control; view permission gets View only; neither hides the action.
  Server remains authoritative; the minted token carries the signed
  access claim.
- Disconnect via confirm dialog (DELETE /api/sessions/:id), invalidates
  the sessions query. List polls every 8s so consent transitions surface.

Passed Code Review (no blockers) 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

@@ -17,7 +17,7 @@ interface NavItem {
 
 const NAV: NavItem[] = [
   { to: "/machines", label: "Machines", Icon: MachinesIcon, enabled: true },
-  { to: "/sessions", label: "Sessions", Icon: SessionsIcon, enabled: false },
+  { to: "/sessions", label: "Sessions", Icon: SessionsIcon, enabled: true },
   { to: "/codes", label: "Codes", Icon: CodesIcon, enabled: false },
   { to: "/users", label: "Users", Icon: UsersIcon, enabled: false },
 ];