[FE-TYPE-004] fe-type: Add type guards for runtime type checking
- Created comprehensive type guard functions (typeGuards.ts) for:
* User, Track, Playlist, Conversation, Message
* Session, AuditLog, Notification
* ApiError, ApiResponse, PaginationData
* Arrays of all entity types
- Added utility type guards:
* isUUID, isEmail, isISO8601Date, isURL
* isNonEmptyString, isPositiveNumber, isNonNegativeNumber
* isPlainObject, isArrayOf, isNotNull, isDefined
* isNumber, isBoolean, isString
- Enables safe type narrowing in TypeScript
- Improves runtime type safety throughout the application
- Comprehensive test suite (44 tests, all passing)
- Allows TypeScript to narrow types safely at runtime
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