Add platform-specific extension install paths

Enhanced extension installation documentation across docs, examples, and the generated install guide. Added Windows, macOS, and Linux-specific paths to help users find the correct installation directory. The dynamic README now shows the machine's actual extensions directory and typical OS paths. Also includes a minor fix removing wide string literal prefix from a path string.

src/app/nebula_controller.cpp

@@ -28,7 +28,7 @@ constexpr size_t kMaxSiteHistoryEntries = 200;
 
 std::filesystem::path GetSiteHistoryPath() {
     const auto user_data = nebula::ui::GetUserDataDirectory();
-    return user_data.empty() ? std::filesystem::path{} : user_data / L"site_history.txt";
+    return user_data.empty() ? std::filesystem::path{} : user_data / "site_history.txt";
 }
 
 std::string ToLowerAscii(std::string value) {

src/extensions/extension_manager.cpp

@@ -274,10 +274,17 @@ void WriteInstallReadme(const std::filesystem::path& extensions_dir) {
         return;
     }
 
+    const std::string dir_utf8 = nebula::platform::PathToUtf8(extensions_dir);
     output << "Nebula Extensions\n"
            << "=================\n\n"
+           << "This machine's install directory:\n"
+           << "  " << dir_utf8 << "\n\n"
            << "Drop an unpacked extension folder here (or copy/paste it into this directory).\n"
            << "Each folder must contain a manifest.json file.\n\n"
+           << "Typical locations by OS:\n"
+           << "  Windows:  %LOCALAPPDATA%\\Nebula\\User Data\\extensions\\\n"
+           << "  macOS:    ~/Library/Application Support/Nebula/User Data/extensions/\n"
+           << "  Linux:    ~/.local/share/Nebula/User Data/extensions/\n\n"
            << "Example layout:\n"
            << "  extensions/\n"
            << "    my-extension/\n"