feat: add theme toggle component and theme store for light/dark mode support

- Implemented ThemeToggle component for switching between light, dark, and system themes.
- Created themeStore for managing theme state and persisting user preferences in localStorage.
- Added utility functions for error message sanitization to prevent sensitive data leakage.
- Developed proxy utility functions for API requests, including template variable replacement.
- Enhanced layout with dark mode styles and smooth transitions for theme changes.
- Updated main layout and page components to integrate theme toggle and improve accessibility.
- Added server-side proxy handling with validation and error sanitization for API requests.

src/lib/utils/error-sanitizer.ts

@@ -0,0 +1,80 @@
+/**
+ * Error message sanitization utility
+ * Prevents sensitive information leakage in error messages
+ */
+
+// Patterns that might indicate sensitive information
+const SENSITIVE_PATTERNS = [
+	// Password/secret related
+	/password|passwd|pwd|secret|token|api[_-]?key|private[_-]?key|auth/i,
+	// File paths
+	/[a-z]:[\\/][^\s]*/i, // Windows paths
+	/\/(?:home|usr|var|etc|root)[\\/][^\s]*/i, // Unix paths
+	// Stack trace patterns
+	/\s+at\s+.*\s+\(\s*[^)]+\s*\)/,
+	/from\s+[^\/\s]+\/[^\/\s]+\.js/,
+	// Internal server details
+	/localhost|127\.0\.0\.1|0\.0\.0\.0|::1/i,
+	// Database connection strings
+	/mongodb|mysql|postgres|redis|sqlite[:+]/i,
+	/aws_access_key_id|aws_secret_access_key/i
+];
+
+/**
+ * Sanitizes an error message to prevent sensitive information leakage
+ *
+ * @param message - The raw error message
+ * @returns A sanitized error message safe to show to users
+ */
+export function sanitizeErrorMessage(message: string): string {
+	if (!message || typeof message !== 'string') {
+		return 'An error occurred';
+	}
+
+	let sanitized = message;
+
+	// Remove or mask sensitive patterns
+	for (const pattern of SENSITIVE_PATTERNS) {
+		sanitized = sanitized.replace(pattern, '[REDACTED]');
+	}
+
+	// Limit length to prevent excessive error messages
+	const MAX_LENGTH = 200;
+	if (sanitized.length > MAX_LENGTH) {
+		sanitized = sanitized.slice(0, MAX_LENGTH) + '...';
+	}
+
+	// Ensure we have a non-empty message
+	if (!sanitized.trim()) {
+		return 'An error occurred while processing your request';
+	}
+
+	return sanitized;
+}
+
+/**
+ * Creates a safe error object for API responses
+ *
+ * @param error - The original error
+ * @returns A sanitized error object
+ */
+export function createSafeError(error: unknown): { error: string; code?: string } {
+	if (error instanceof Error) {
+		return {
+			error: sanitizeErrorMessage(error.message),
+			code: error.name || 'ERROR'
+		};
+	}
+
+	if (typeof error === 'string') {
+		return {
+			error: sanitizeErrorMessage(error),
+			code: 'ERROR'
+		};
+	}
+
+	return {
+		error: 'An unexpected error occurred',
+		code: 'UNKNOWN_ERROR'
+	};
+}