security: wipe RPC creds, lock down generated conf, auto-clear secret clipboard (audit #4-6)

- rpc_client: wipe the plaintext "user:password" temporary with sodium_memzero after
  base64-encoding it into the auth header (std::string doesn't zero its buffer on
  destruction).
- connection: the auto-generated DRAGONX.conf holds rpcuser/rpcpassword in plaintext but
  was written with the default umask (often world-readable 0644). Restrict it to owner
  read/write after creation so another local user can't read the credentials.
- app: copying a seed phrase / private key to the clipboard now arms an auto-clear —
  App::copySecretToClipboard() copies the secret and, after 45s, wipes the clipboard IF it
  still holds that secret (compared via a stored hash, never the plaintext). Wired into the
  lite first-run wizard's seed Copy and the Settings export-secret Copy, with a
  "clipboard auto-clears in 45s" notice. pumpSecretClipboardClear() runs each frame.

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

src/app.h

@@ -389,6 +389,13 @@ public:
     bool hasTransactionSendProgress() const { return send_progress_active_ || send_submissions_in_flight_ > 0 || !pending_opids_.empty(); }
     std::string transactionSendProgressText() const;
     std::string transactionRefreshProgressText() const;
+
+    // Copy a SECRET (seed phrase, private key) to the clipboard and arm an auto-clear: after a
+    // short delay the clipboard is wiped IF it still holds this secret (so we don't clobber
+    // something the user copied afterwards). Only a hash of the secret is retained, never the
+    // plaintext. Call pumpSecretClipboardClear() each frame to action the clear.
+    void copySecretToClipboard(const std::string& secret);
+    void pumpSecretClipboardClear();
     bool isTransactionRefreshInProgress() const {
         return network_refresh_.jobInProgress(services::NetworkRefreshService::Job::Transactions);
     }
@@ -512,6 +519,9 @@ private:
     int daemon_wait_attempts_ = 0;
     bool daemon_start_error_shown_ = false;
     int daemon_last_seen_crashes_ = 0;  // surface each new embedded-daemon crash reason once
+    // Auto-clear for secrets copied to the clipboard. Only a hash of the copied secret is kept.
+    std::uint64_t clipboard_secret_hash_ = 0;
+    double clipboard_clear_deadline_ = 0.0;
     float loading_timer_ = 0.0f;  // spinner animation for loading overlay
 
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