feat: add Map Discovery page with responsive layout and loading states (Closes #1138)

[waterWang]

Summary

This PR adds the Map Discovery Page at /discover/map, providing the page layout and placeholder container for the interactive event map (to be integrated in #1139).

Changes

• New route: /discover/map with SEO metadata via buildMetadata
• New component: MapDiscoveryPage — responsive layout with header, placeholder map container, and loading states
• Loading state: Brief loading animation using LoadingBar component, then shows the map placeholder ready for interactive integration
• Tests: 3 test cases covering loading state, heading rendering, and completion state

Acceptance Criteria

• Route renders successfully at /discover/map
• Layout is responsive (mobile + desktop via Tailwind responsive classes)
• Placeholder map container is ready for ## FRONTEND: Integrate Interactive Event Map #1139 integration (with data-testid="map-placeholder")
• Loading states display correctly (initial loading bar → placeholder)
• Tests pass (3/3)

Dependencies

• Closes ## FRONTEND: Create Map Discovery Page #1138
• Prerequisite for ## FRONTEND: Integrate Interactive Event Map #1139 (Interactive Event Map)

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