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.

docs/cross-platform.md

@@ -169,6 +169,8 @@ Shared logic in `src/ui/paths.cpp` writes under:
 | macOS | `~/Library/Application Support/Nebula/User Data` |
 | Linux | `$XDG_DATA_HOME/Nebula/User Data` or `~/.local/share/Nebula/User Data` |
 
+Unpacked extensions load from `<user data>/extensions/` on every OS (`GetExtensionsDirectory()`). Settings → Plugins shows the live path.
+
 Platform-specific discovery lives in `src/platform/*/paths_*.cpp`.
 
 ## GPU / Chromium flags

examples/extensions/hello-nebula/README.txt

@@ -1,8 +1,13 @@
 Copy this folder into Nebula's extensions directory to install:
 
-  %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\
+  Windows:  %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\
+  macOS:    ~/Library/Application Support/Nebula/User Data/extensions/hello-nebula/
+  Linux:    ~/.local/share/Nebula/User Data/extensions/hello-nebula/
+            (or $XDG_DATA_HOME/Nebula/User Data/extensions/hello-nebula/)
 
-Then open Settings > Plugins and click Reload Plugins (or restart Nebula).
+Or open Settings > Plugins — the install path is shown there.
+
+Then click Reload Plugins (or restart Nebula).
 
 manifest.json fields:
   id, name, version, description, enabled

examples/extensions/return-youtube-dislike/README.txt

@@ -20,7 +20,12 @@ Install
 -------
 Copy this folder to:
 
-  %LOCALAPPDATA%\Nebula\User Data\extensions\return-youtube-dislike\
+  Windows:  %LOCALAPPDATA%\Nebula\User Data\extensions\return-youtube-dislike\
+  macOS:    ~/Library/Application Support/Nebula/User Data/extensions/return-youtube-dislike/
+  Linux:    ~/.local/share/Nebula/User Data/extensions/return-youtube-dislike/
+            (or $XDG_DATA_HOME/Nebula/User Data/extensions/return-youtube-dislike/)
+
+Or open Settings > Plugins — the install path is shown there.
 
 Open Settings > Plugins > Reload Plugins, then open a YouTube video.
 

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"