fix: resolve did:dht compilation errors

- Simplify DHT encoding: use JSON instead of DNS packets (drop simple-dns)
- Fix mainline crate API: SigningKey takes 32 bytes, get_mutable returns Result
- Add missing dht_did field to IdentityRecord constructor
- Store DID Document as JSON in DHT (DNS encoding deferred)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

neode-ui/src/components/SplashScreen.vue

@@ -267,7 +267,15 @@ watch([showWelcome, showLogo], ([welcome, logo]) => {
 })
 
 // Check if user has seen intro
-const seenIntro = localStorage.getItem('neode_intro_seen') === '1'
+// Also detect returning users who cleared cache: if we're on a /dashboard route,
+// the backend session is still active so the user has been here before.
+const storedSeenIntro = localStorage.getItem('neode_intro_seen') === '1'
+const isOnDashboard = window.location.pathname.startsWith('/dashboard')
+const seenIntro = storedSeenIntro || isOnDashboard
+// Persist the flag so subsequent loads don't re-check
+if (!storedSeenIntro && isOnDashboard) {
+  localStorage.setItem('neode_intro_seen', '1')
+}
 
 function onIntroLogoHover() {
   introLogoHover.value = true