feat: clickable robot cards with detail modal

[xdnaimino]

Fixes #6

Changes

• Card.js: Add onSelect prop with role=button, tabIndex=0, keyboard handlers (Enter/Space), pointer cursor, aria-label. Fix robohash URL from ?50x50 to ?size=200x200
• CardList.js: Accept and forward onSelect callback to each Card
• App.js: Add selectedRobot state, onSelectRobot/onCloseModal methods, pass onSelect to CardList, render RobotModal when selected
• RobotModal.js (new): Centered modal overlay with large avatar, name/email/ID, Close button. Accessibility: focus trap, Esc to close, backdrop click to close, aria-modal, role=dialog, body scroll lock
• RobotModal.test.js (new): 11 tests covering rendering, a11y attributes, close button, backdrop click, Escape key, scroll lock, and null robot

Acceptance Criteria

• Clicking a card opens a modal showing the large avatar, name, email, and ID
• Cards show cursor: pointer, are focusable via Tab, activatable with Enter/Space
• Modal closes via: Close button, backdrop click, and Escape key
• Background scroll locked while modal open, focus moves into modal, aria-modal/role=dialog present
• Selecting a different card while modal is open updates the modal content
• No console warnings/errors; existing search filtering still works

Evidence

• All 24 tests pass (Card.test.js: 8, SearchBox.test.js: 5, RobotModal.test.js: 11)
• Production build succeeds

[syntaro-bot]

🔄 Syntaro quality gate: waiting for CI checks to pass before marking this PR ready to merge…

[syntaro-bot]

🔄 Syntaro requested review from: @Malek-Ghorbel