feat(settings): add appearance and custom theme support

Introduce an appearance preference with modern, classic, and custom
modes and a persisted custom theme palette. Add helpers to load,
persist, and apply appearance and custom colors, and wire them into the
app settings lifecycle. Implement a comprehensive light theme and
appearance presets using CSS variables so custom palettes are applied
consistently across the UI, and include appearance in analytics when
settings are saved.

- Persist and apply appearance and custom color palette to :root
- Add muted light theme plus classic/modern presets and custom mapping
- Include appearance and customTheme in settings save and analytics payload

src/lib/types.ts

@@ -13,8 +13,16 @@ 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 AppAppearance = "modern" | "classic" | "custom";
 export type AppLanguage = "en" | "de";
 
+export interface CustomThemeColors {
+  background: string;
+  surface: string;
+  accent: string;
+  text: string;
+}
+
 export interface CommitAiStatus {
   phase: CommitAiPhase;
   model_id: string | null;