feat(diagnostics): add "Copy diagnostics" + "Open log folder" actions (Foundation QoL)

Settings (logging section) gains two support-friendly actions, now that the logging
foundation actually produces logs (W7-2):

- Open log folder: opens the config dir (Platform::openFolder) so users can find
  dragonx-debug.log / dragonx-crash.log.

- Copy diagnostics: copies a plaintext support snapshot to the clipboard via the new
  App::buildDiagnosticsReport() — version, build variant, platform, connection status,
  active wallet path + existence + size, encryption/lock state, sync heights, and (full-
  node) daemon status/running/crash-count/lastError, plus the log paths. No secrets.

Build-clean; ctest 1/1. Remaining QoL: persistent alert history, a daemon/RPC error
banner, and the W6-2 refresh-staleness badge. See docs/wallet-hardening.md.

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

src/app.h

@@ -147,6 +147,10 @@ public:
     bool isLiteBuild() const { return wallet::isLiteBuild(walletCapabilities()); }
     bool supportsEmbeddedDaemon() const { return wallet::supportsEmbeddedDaemon(walletCapabilities()); }
     bool supportsFullNodeLifecycleActions() const { return wallet::supportsFullNodeLifecycleActions(walletCapabilities()); }
+
+    // W7 QoL: a plaintext support-diagnostics snapshot (version, variant, daemon/RPC/wallet/log state)
+    // for the "Copy diagnostics" action. Contains no secrets.
+    std::string buildDiagnosticsReport();
     bool supportsSoloMining() const { return wallet::supportsSoloMining(walletCapabilities()); }
     bool supportsPoolMining() const { return wallet::supportsPoolMining(walletCapabilities()); }
     bool supportsLiteBackend() const { return wallet::supportsLiteBackend(walletCapabilities()); }