Feat/challenge matching_brackets

[nank1ro]

Summary

• Adds the "Matching brackets" coding challenge (difficulty 3): given a string of brackets, determine whether every opening bracket has a matching, correctly-nested closing bracket.
• Implemented as a run-code (type 1) challenge in all 6 supported languages: C, Dart, JavaScript, Kotlin, Python, Swift.
• Translated to all 12 locales (en, de, es, fr, hi, it, ja, ko, pl, pt, ru, zh) — 72 challenge files + 72 data.json entries + assets/challenges/matching_brackets.svg.
• The German locale was previously missing entirely for this repo's challenge set; this PR adds it by translating the learner-visible text of each en file while keeping code blocks, harnesses, and --err-tN-- markers byte-identical to en, matching how it/es/fr already handle the same challenge.

Checks run

• Validator: pass.
• Tester: all 6 en type-1 challenge files pass against the codecompiler-8081 container (c 3s, dart 4s, javascript 1s, kotlin 2s, python 1s, swift 3s). The 6 newly-authored de files also pass.
• scripts/check_locales.py: 0 problems across 12 en files x 11 locales.
• scripts/check_string_literals.py: 0 unterminated string literals.
• curriculum.json and _theory.md regenerated — both are legitimately unchanged (curriculum.json does not index challenge names, and theory_creator emits no _theory.md under challenges/), so both generators produced byte-identical output.

Known issues

None.