feat: add high contrast mode for accessibility

- Add HighContrast variant to Theme enum in Rust backend
- Add high-contrast CSS theme with pure black/white for max contrast
- Use bright saturated colors for syntax highlighting
- Add High Contrast button to theme selector with accessibility tooltip
- Follows WCAG guidelines for contrast ratios

Closes #20

src/lib/stores/config.ts

@@ -1,7 +1,7 @@
 import { writable, derived } from "svelte/store";
 import { invoke } from "@tauri-apps/api/core";
 
-export type Theme = "dark" | "light";
+export type Theme = "dark" | "light" | "high-contrast";
 
 export interface HikariConfig {
   model: string | null;