Fix test build errors, update CI version, add Snap/Flatpak/AppStream packaging

- DoS_tests: remove extra arg from VerifySignature calls (5 -> 4 params)
- accounting_tests: int64 -> int64_t for modern compilers
- CI: bump VERSION 5.3.5 -> 5.3.6
- Snap/Flatpak: fix asset URLs to match CI naming convention
- Add AppStream metainfo for store listings
- Add DNS2 seed node setup guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/test/accounting_tests.cpp

@@ -8,7 +8,7 @@
 BOOST_AUTO_TEST_SUITE(accounting_tests)
 
 static void
-GetResults(CWalletDB& walletdb, std::map<int64, CAccountingEntry>& results)
+GetResults(CWalletDB& walletdb, std::map<int64_t, CAccountingEntry>& results)
 {
     std::list<CAccountingEntry> aes;
 
@@ -27,7 +27,7 @@ BOOST_AUTO_TEST_CASE(acc_orderupgrade)
     std::vector<CWalletTx*> vpwtx;
     CWalletTx wtx;
     CAccountingEntry ae;
-    std::map<int64, CAccountingEntry> results;
+    std::map<int64_t, CAccountingEntry> results;
 
     ae.strAccount = "";
     ae.nCreditDebit = 1;