Add JSDoc and inline documentation to App.jsx

[Copilot]

Adds comprehensive documentation to frontend/src/App.jsx covering component functionality, state management, and hero comparison logic.

Documentation Added

• File-level: Purpose and functionality overview of the Superhero Comparison app
• Component & Functions: JSDoc annotations for App component and all 7 helper functions with parameter/return types
• Inline comments: State variables, selection logic (toggle/add/replace behavior), and stat comparison algorithm

Example

/**
 * Calculates the winner between two heroes by comparing their power statistics.
 * Each stat category (intelligence, strength, speed, etc.) is scored individually,
 * and the hero with more winning categories is declared the overall winner.
 * 
 * @param {Object} hero1 - The first hero to compare
 * @param {Object} hero2 - The second hero to compare
 * @returns {{winner: Object|null, score: string}} Object containing winner (hero object or null for tie) and score string in 'X-Y' format
 */
const calculateWinner = (hero1, hero2) => {
  // All stat categories used for comparison
  const stats = ['intelligence', 'strength', 'speed', 'durability', 'power', 'combat'];
  // ...
}

No functional changes.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add documentation to frontend/src/App.jsx</issue_title>
<issue_description>Add documentation to frontend/src/App.jsx:

• Overall file doc
• Function docs
• Inline docs</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add documentation to frontend/src/App.jsx #4

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