Fix ESLint warnings and suppress test-specific linting rules

- Allow @typescript-eslint/no-explicit-any in test files for vitest mocks
- Use 'unknown' type instead of 'any' in dates.ts isValidDate function
- npm run lint now passes with zero errors

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/utils/dates.ts

@@ -41,7 +41,7 @@ export function parseInputDate(value: string): Date {
 /**
  * Type guard: check if a value is a valid date.
  */
-export function isValidDate(date: any): boolean {
+export function isValidDate(date: unknown): boolean {
   return date instanceof Date && !isNaN(date.getTime())
 }