Merge Workstream H: device pairing UI + live dashboard

Resolved conflicts in App.tsx/SeancePage.tsx (both F and H added new
routes/nav links, kept both) and i18n files (both added sibling top-level
keys — inventory + devices, fixed nesting after conflict markers removed).

Also fixed a real integration gap: DevicesPage.test.tsx's mock User object
predated Workstream F's unlocks/essence additions to the User type (the
two workstreams built in parallel isolation and couldn't see each other's
changes). 269/269 frontend tests pass, tsc clean.

frontend/src/pages/SeancePage.tsx

@@ -260,6 +260,10 @@ function SeanceSession({ username }: { username: string }) {
             {t('nav.inventory', { defaultValue: 'inventory' })}
           </Link>
 
+          <Link to="/devices" className="seance-codex-link">
+            {t('nav.devices', { defaultValue: 'devices' })}
+          </Link>
+
           <span className="seance-user" title={username}>
             {username}
           </span>