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/lib/types.ts

@@ -11,6 +11,7 @@ export type CommitAiPhase = "idle" | "loading" | "ready" | "error";
 export type CommitAiProvider = "local" | "openai" | "anthropic" | "custom";
 export type CommitAiLocalProfile = "fast" | "balanced" | "detailed";
 export type AppTheme = "system" | "light" | "dark";
+export type AppLanguage = "en" | "de";
 
 export interface CommitAiStatus {
   phase: CommitAiPhase;