Add number-to-text and text-to-number exercises for numbers 0-20

[Null000]

Summary

This PR introduces two new exercise categories for learning number representation: converting numbers to their text form and vice versa. These exercises cover numbers 0-20 and are designed as foundational exercises for first-grade students.

Key Changes

• New module src/numberText.ts: Implements problem generation for two new categories:

  • NumberToText_Twenty: Displays a number (0-20) and asks students to select the correct word form
  • TextToNumber_Twenty: Displays a number word and asks students to select the correct numeric value

• Problem generation logic:

  • generateNumberToText(): Creates 21 problems (one per number 0-20) with 3 random distractors for multiple-choice answers
  • generateTextToNumber(): Creates 21 problems with numeric answers (no multiple-choice options)
  • Implements Fisher-Yates shuffle algorithm for randomizing answer options

• Updated src/common.ts:

  • Added two new Category enum values: NumberToText_Twenty and TextToNumber_Twenty
  • Created new NumberText category group containing both new categories
  • Added both categories to yearGroupsSl for first-grade ("1. razred") curriculum

• Updated src/translations.ts:

  • Added English translations for all number words (0-20) and category labels
  • Added Slovenian translations for all number words (0-20) and category labels
  • Added new group name translations: "Number & Text" (English) and "Številke in besede" (Slovenian)

• Updated src/problem.ts:

  • Imported and integrated the new numberText module into the problem generation system

Implementation Details

• Number words are retrieved dynamically using the current language setting via getCurrentLanguage()
• The NumberToText_Twenty category generates multiple-choice problems with 3 distractors randomly selected from the 0-20 range
• The TextToNumber_Twenty category generates free-response problems (answer is numeric, no options provided)
• Both categories follow the existing problem structure and naming conventions

https://claude.ai/code/session_01RTuNHdUoSZoAnjspngKdkt