feat(language): implement internationalization and help overlay

This update introduces comprehensive language support (English/German) across the application, enabling localization for UI elements and settings dialogs. It also adds a dedicated Help Overlay component with detailed guides on using Gitty's core Git features.

- Adds language selection to App Settings
- Implements German translations in key areas
- Introduces global help documentation accessible via Ctrl+/

src/app.css

@@ -3083,6 +3083,10 @@
     white-space: nowrap;
   }
 
+  .settings-segmented.settings-language {
+    grid-template-columns: repeat(2, minmax(0, 1fr));
+  }
+
   .settings-toggle-row {
     display: grid;
     grid-template-columns: auto minmax(0, 1fr);