feat(challenges): add Rust coding challenges (#1)

[fguzman-stack]

🎯 What does this PR do?

Adds 10 Rust coding challenges (ids 201-210) to ChallengesData.kt covering ownership, borrowing, Result/Option, slices, iterators, the ? operator and #[derive(Debug)]. Mixes TRIVIA, DEBUG and PRINT types. Also wires Rust as a selectable language in the UI (icon + selector + badges + focus topics).

Closes #1

🧩 Challenges added

• Ownership / move semantics
• Borrow conflict (E0502)
• Moved value (E0382)
• Result<T,E> and the ? operator
• Option::unwrap_or, string slices, iterator pipelines, #[derive(Debug)]

All codeSnippets were validated with rustc 1.91 locally (compiling ones build & print the expected output; the two "DEBUG" ones reproduce the exact E0502/E0382 compile errors).

🖼️ UI wiring

• ic_lang_rust.xml (rust-orange gear vector) + mapping in TopicIcons.kt
• Added "Rust" to language selector (SettingsLanguageScreen), badges list (LearnScreen, key RUST), and Focus topics

✅ Before you submit

• ./gradlew testDebugUnitTest passes locally (BUILD SUCCESSFUL)
• No Room schema changes (language is a TEXT key, no migration needed)
• No secrets/keystores committed
• Conventional commit title

🧪 How to test

1. Settings → Lenguajes y temas → "Lenguaje Principal" → pick Rust
2. Aprender → "Retos Normales": solve Rust challenges (trivia/print/debug)
3. Insignias tab shows a Rust progress row
4. Focus → topic list includes Rust