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/ui/windows/export_all_keys_dialog.cpp

@@ -9,6 +9,7 @@
 #include "../../rpc/rpc_worker.h"
 #include "../../util/i18n.h"
 #include "../../util/platform.h"
+#include "../../util/export_filename.h"
 #include "../notifications.h"
 #include "../schema/ui_schema.h"
 #include "../material/draw_helpers.h"
@@ -160,8 +161,11 @@ void ExportAllKeysDialog::render(App* app)
                 if (s_include_t) {
                     for (const auto& a : state.t_addresses) t_addrs.push_back(a.address);
                 }
-                std::string filename(s_filename);
-                
+                // Sanitize the typed filename (basename only, no traversal) and reflect it back in the
+                // buffer so the user sees exactly what gets written. Mirrors the CSV export dialog.
+                std::string filename = util::sanitizeExportFilename(s_filename, ".txt", "dragonx_keys.txt");
+                snprintf(s_filename, sizeof(s_filename), "%s", filename.c_str());
+
                 // Run all key exports on worker thread
                 if (app->worker()) {
                     app->worker()->post([rpc = app->rpc(), z_addrs, t_addrs, filename]() -> rpc::RPCWorker::MainCb {