feat(challenges): add Go coding challenges (#2)

[fguzman-stack]

🎯 What does this PR do?

Adds 10 Go coding challenges (ids 211-220) to ChallengesData.kt covering arrays vs slices, append semantics, the "declared and not used" rule, goroutines + sync.WaitGroup, defer ordering, exported identifiers, the idiomatic error-handling convention and variable shadowing. Mixes TRIVIA, DEBUG and PRINT types. Also wires Go as a selectable language in the UI.

Closes #2

🧩 Challenges added

• Arrays vs slices (value copy vs view)
• len/cap on subslices and append
• declared and not used compile error
• Waiting for goroutines with sync.WaitGroup
• defer LIFO order + argument evaluation timing
• Exported (uppercase) visibility
• Idiomatic if err != nil

All codeSnippets were validated with go run on go1.26 locally: the PRINT ones produce exactly the documented output (3 4, b/a, 2 3, 2/1, 210) and the DEBUG one reproduces the real declared and not used: x error.

🖼️ UI wiring

• ic_lang_go.xml (gopher face vector) + mapping in TopicIcons.kt
• Added "Go" to language selector (SettingsLanguageScreen), badges list (LearnScreen, key GO), 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 Go
2. Aprender → "Retos Normales": solve Go challenges (trivia/print/debug)
3. Insignias tab shows a Go progress row
4. Focus → topic list includes Go

📝 Note: independent from #4 (branched off master). Whoever merges second will need a trivial conflict resolution in ChallengesData.kt / the language lists (both PRs touch the same append points).