feat(ui): Implement theme persistence and system preference support

Adds comprehensive theme management across the application, allowing users to save their preferred color scheme (light/dark) or automatically follow the operating system's settings. This required updating state management in App.svelte to handle theme loading and applying changes dynamically via CSS variables.

The styling layer was significantly updated with new CSS variables and rules for both dark and light modes, ensuring visual consistency across all components like dialogs, buttons, and sidebars.

- Added logic to detect system color scheme preference
- Implemented full support for persistent light/dark mode switching
- Overhauled global CSS variables for improved theme adaptability

src/lib/types.ts

@@ -10,6 +10,7 @@ export type FileStatusKind =
 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 interface CommitAiStatus {
   phase: CommitAiPhase;