security: payment-URI amount validation, export filename sanitize, chat crash-guard, HTTPS-only, conf 0600, pool auto-lock

Six bounded defensive fixes:
- Payment-URI amount is now strictly validated (reject NaN/inf/sign/garbage,
  <=8 decimals, 0..21,000,000) at parse time, so a crafted QR/URI can't inject a
  bogus amount into the Send form. The z_sendmany call the user confirms is
  unchanged; this is input validation only.
- export-all-keys filename is sanitized (basename-only, no traversal) via a
  shared util::sanitizeExportFilename (factored out of the CSV export dialog).
- Chat append()/upsert() wrap the strict serialize in try/catch so a malformed
  (invalid-UTF-8, attacker-influenceable) body is skipped, not crashed on —
  matching appendBatch's existing drop behavior.
- http_download (used by the updaters + pool stats) restricts transfers AND
  redirects to HTTPS and bounds the download path's redirects, so a redirect
  can't downgrade to http:// or another scheme.
- The default DRGX.conf (rpcuser/rpcpassword) is written 0600 from the start via
  writeFileAtomically, closing the brief world-readable ofstream+chmod window.
- Auto-lock is suppressed only for SOLO mining (needs the wallet), not pool
  mining (pays an external address), which was holding the wallet open forever.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/rpc/connection.cpp

@@ -460,15 +460,14 @@ bool Connection::createDefaultConfig(const std::string& path)
         return false;
     }
     
-    std::ofstream file(path);
-    if (!file.is_open()) {
-        DEBUG_LOGF("Failed to create config file: %s\n", path.c_str());
-        return false;
-    }
-    
+    // Build the content in memory, then write it 0600 from the start (writeFileAtomically with
+    // restrictPermissions) — the old ofstream+chmod left a brief world-readable window during which
+    // another local user could read the freshly-generated rpcuser/rpcpassword.
+    std::ostringstream file;
+
     // Get the data directory for exportdir
     std::string dataDir = getDefaultDataDir();
-    
+
     file << "# DragonX configuration file\n";
     file << "# Auto-generated by DragonX Wallet\n";
     file << "\n";
@@ -485,18 +484,10 @@ bool Connection::createDefaultConfig(const std::string& path)
     file << "addnode=seed.dragonx.is\n";
     file << "addnode=node1.dragonx.is\n";
     file << "addnode=node5.dragonx.is\n";
-    
-    file.close();
 
-    // The file holds the freshly-generated rpcuser/rpcpassword in plaintext. ofstream creates it
-    // with the process umask (typically world-readable 0644), so restrict it to owner read/write
-    // before another local user can read the credentials.
-    {
-        namespace fs = std::filesystem;
-        std::error_code ec;
-        fs::permissions(path, fs::perms::owner_read | fs::perms::owner_write,
-                        fs::perm_options::replace, ec);
-        if (ec) DEBUG_LOGF("Could not restrict config permissions on %s: %s\n", path.c_str(), ec.message().c_str());
+    if (!util::Platform::writeFileAtomically(path, file.str(), /*restrictPermissions=*/true)) {
+        DEBUG_LOGF("Failed to create config file: %s\n", path.c_str());
+        return false;
     }
 
     DEBUG_LOGF("Created default config file: %s\n", path.c_str());