feat: add named exercise groups to web-evals

[roomote]

This PR adds support for predefined named groups of eval exercises that can be quickly selected in the web-evals app.

Changes

• Added exercise-groups.json file to store predefined exercise groups
• Created a "Hard" group containing 26 challenging exercises across Go, Java, JavaScript, Python, and Rust
• Added server action getExerciseGroups() to load exercise groups
• Updated the NewRun component to display exercise group buttons when "Some" exercises are selected
• Clicking a group button automatically selects all exercises in that group

UI Enhancement

When users select "Some" exercises instead of "All", they now see predefined group buttons above the exercise selector. Clicking "Hard (26)" will instantly select all 26 exercises in the Hard group.

This feature makes it easier to run specific sets of exercises without manually selecting each one.

---

Important

Adds predefined exercise groups to web-evals, allowing quick selection of exercise sets in the NewRun component.

• Behavior:
  • Adds exercise-groups.json to define predefined exercise groups.
  • Implements getExerciseGroups() in exercise-groups.ts to fetch exercise groups.
  • Updates NewRun component to show group buttons when "Some" exercises are selected.
  • Clicking a group button selects all exercises in that group.
• UI Enhancement:
  • Displays predefined group buttons in NewRun when "Some" exercises are selected.
  • "Hard (26)" button selects 26 challenging exercises across multiple languages.

^{This description was created by for 9861ca6. You can customize this summary. It will automatically update as commits are pushed.}

[ellipsis-dev]

Use the i18n function (e.g., t('...')) for the success toast message instead of inline English strings.

Suggested change

	toast.success(`Selected "${groupName}" exercise group with ${group.exercises.length} exercises`)
	toast.success(t('Selected "{{groupName}}" exercise group with {{count}} exercises', { groupName, count: group.exercises.length }))

^{This comment was generated because it violated a code review rule: irule_C0ez7Rji6ANcGkkX.}

[daniel-lxs]

For visibility @cte