feat(neode-ui): seed & entropy explainer page at /entropy/ + link from Backup settings

Standalone static guide (same pattern as /architecture/) covering how the
master seed entropy is drawn (explicit OsRng, sealed KeyGenRng allowlist,
degenerate-draw refusal, CSPRNG readiness ledger), how it is stored
(Argon2 + ChaCha20-Poly1305 envelope), the full derivation tree (HKDF
labels, NIP-06, LND aezeed one-way gate, second-order keys), what is NOT
seed-derived, every failure/fallback path, and the restore flow — in
paired layman/technical language. Linked from the Recovery-phrase card
in Settings → Backup. CSP-safe: no inline scripts.

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

neode-ui/src/views/settings/BackupSection.vue

@@ -289,6 +289,15 @@ defineExpose({ loadBackups })
           (and 2FA code, if enabled). Only reveal it somewhere private — anyone with these
           words controls this node.
         </p>
+        <a
+          href="/entropy/"
+          target="_blank"
+          rel="noopener"
+          class="inline-flex items-center gap-1 mt-2 text-sm text-orange-300/90 hover:text-orange-200 transition-colors"
+        >
+          How your seed &amp; keys work — the full guide
+          <svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10 6H6a2 2 0 00-2 2v10a2 2 0 002 2h10a2 2 0 002-2v-4M14 4h6m0 0v6m0-6L10 14"/></svg>
+        </a>
       </div>
       <button
         type="button"