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/util/payment_uri.cpp

@@ -10,6 +10,29 @@
 #include <algorithm>
 #include <cctype>
 #include <cstdio>
+#include <cmath>
+
+namespace {
+// Strictly parse a decimal DRGX amount from an (attacker-influenceable) payment URI / QR: digits with at
+// most one '.', at most 8 fractional digits, in [0, 21000000]. Rejects empty, NaN/inf, signs, whitespace
+// and trailing garbage that std::stod would otherwise accept — so a crafted URI can't inject a bogus
+// amount into the Send form.
+bool parseStrictAmount(const std::string& s, double& out) {
+    if (s.empty() || s.size() > 32) return false;
+    int dots = 0, digits = 0, frac = 0; bool afterDot = false;
+    for (char c : s) {
+        if (c == '.') { if (++dots > 1) return false; afterDot = true; }
+        else if (c >= '0' && c <= '9') { ++digits; if (afterDot && ++frac > 8) return false; }
+        else return false;
+    }
+    if (digits == 0) return false;
+    double v = 0.0;
+    try { v = std::stod(s); } catch (...) { return false; }
+    if (!std::isfinite(v) || v < 0.0 || v > 21000000.0) return false;
+    out = v;
+    return true;
+}
+}  // namespace
 
 namespace dragonx {
 namespace util {
@@ -141,11 +164,13 @@ PaymentURI parsePaymentURI(const std::string& uri)
             std::transform(key.begin(), key.end(), key.begin(), ::tolower);
             
             if (key == "amount" || key == "amt") {
-                try {
-                    result.amount = std::stod(value);
-                } catch (...) {
-                    // Invalid amount, keep as 0
+                double a = 0.0;
+                if (!parseStrictAmount(value, a)) {
+                    result.error = "Invalid amount";
+                    result.valid = false;
+                    return result;   // reject the whole URI rather than silently treating it as 0
                 }
+                result.amount = a;
             } else if (key == "label") {
                 result.label = value;
             } else if (key == "memo") {
@@ -174,7 +199,7 @@ PaymentURI parsePaymentURI(const std::string& uri)
     }
     
     // Validate amount if present
-    if (result.amount < 0) {
+    if (!std::isfinite(result.amount) || result.amount < 0 || result.amount > 21000000.0) {
         result.error = "Invalid negative amount";
         return result;
     }