US-07: Code editor surface and problems integrations

[Copilot]

This PR introduces a real code editing surface in challenge-workspace for efficient code writing with visual feedback (gutter, active line, basic syntax highlighting), keeping code editable per language from the feature state.
Additionally, a decoupled contract is defined to allow future replacement of the internal editor without breaking container integration.

What’s included

• Decoupled Editor UI (code-editor):

  • New standalone component with explicit API: code, language, codeChange.
  • Renders line numbers, active line (cursor/scroll), and basic syntax highlighting per language.
  • Surface is ready for future editor implementation swaps.

• Feature State (challenge-workspace.state):

  • New state with selectedLanguage, codeByLanguage, computed code, and language catalog.
  • Loads starter code per language and preserves edits per language when switching.

• Types and Contracts (challenge.types):

  • Adds ChallengeLanguage, challengeLanguages, labels, starter templates, and isChallengeLanguage type guard.
  • Removes dependency on loose strings in selector/state.

• Container Integration:

  • challenge-workspace replaces the static <pre><code> block with <app-code-editor>.
  • Language selector is connected to state and starter code.
  • Now also connects <app-problem-content> and <app-code-editor> in the workspace layout, using the shared problem content component for the problem panel.

• Behavior Coverage:

  • New specs for:
    • Editor (lines, change emission, active line, highlight)
    • State (starter per language, edit persistence per language)
    • Container (selector/editor/state synchronization)

Example usage

<app-problem-content></app-problem-content>
<app-code-editor
  [code]="challengeWorkspaceState.code()"
  [language]="challengeWorkspaceState.selectedLanguage()"
  (codeChange)="onCodeChange($event)"
></app-code-editor>