refactor(audit): unify config-dir resolution through Platform::getConfigDir

Settings::getDefaultPath and AddressBook::getDefaultPath each hand-rolled the
same per-platform #ifdef (Win SHGetFolderPath / macOS Application Support /
XDG ~/.config) with the DRAGONX_APP_NAME variant suffix. Route both through
util::Platform::getConfigDir() instead — one place owns the split — and add
the missing __APPLE__ branch to getConfigDir so it too returns the
macOS-native ~/Library/Application Support/<AppName>.

This also fixes a latent macOS inconsistency: transaction_history_cache
already resolves via getConfigDir(), which lacked the __APPLE__ branch, so on
macOS the tx cache landed in ~/.config while settings/addressbook went to
~/Library/Application Support. Now all three agree. No migration needed:
Linux/Windows paths are unchanged, and macOS packaging is not yet shipped.
Removes ~50 lines of duplicated #ifdef.

Full-node + Lite build clean; ctest 1/1; hygiene clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/util/platform.cpp

@@ -245,6 +245,9 @@ std::string Platform::getConfigDir()
         return std::string(path) + "\\" DRAGONX_APP_NAME "\\";
     }
     return getHomeDir() + "\\AppData\\Roaming\\" DRAGONX_APP_NAME "\\";
+#elif defined(__APPLE__)
+    // macOS-native config location (matches what settings.json / addressbook.json use).
+    return getHomeDir() + "/Library/Application Support/" DRAGONX_APP_NAME "/";
 #else
     return getHomeDir() + "/.config/" DRAGONX_APP_NAME "/";
 #endif