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/app_security.cpp

@@ -791,12 +791,11 @@ void App::checkAutoLock() {
     if (capture_mode_) return;  // don't auto-lock while a UI sweep forces the encrypted/locked state
     if (!state_.isEncrypted() || state_.isLocked()) return;
 
-    // Don't auto-lock while mining — mining is a long-running intentional
-    // operation and locking the wallet on the daemon side stops solo mining.
-    bool miningActive = state_.mining.generate
-                        || (xmrig_manager_ && xmrig_manager_->isRunning());
-    if (miningActive) {
-        // Keep resetting the idle timer so we don't lock the instant mining stops
+    // Don't auto-lock while SOLO mining — it runs in the daemon and locking the wallet there would stop
+    // it (and it shields matured rewards, which needs the wallet). Pool mining pays an external address
+    // and needs no unlocked wallet, so it must NOT hold the wallet open indefinitely.
+    if (state_.mining.generate) {
+        // Keep resetting the idle timer so we don't lock the instant solo mining stops.
         idle_.noteActivity();
         return;
     }