feat: error boundaries for landing and map pages

[benzy018]

Summary

Add Next.js App Router error.tsx files for the root and /map routes so any render-time exception shows a styled fallback with a "Try again" retry button instead of crashing the page.

Changes

File	Purpose
Frontend/src/app/error.tsx	Global error boundary (GlobalError) – catches errors in any route
Frontend/src/app/map/error.tsx	Map-specific error boundary (MapError) – scoped fallback for the map page
Frontend/src/app/error.test.tsx	5 colocated Vitest tests for GlobalError
Frontend/src/app/map/error.test.tsx	6 colocated Vitest tests for MapError

Acceptance criteria

• Throw a render-error in a test component → fallback renders (verified via Vitest with a mock error prop)
• Fallback includes a styled "Try again" retry button that calls Next.js reset()
• Dark theme (bg-[#111827]) and indigo accent (#6366f1) match project conventions
• 11/11 tests pass (vitest run)

Testing

cd Frontend && npm ci && ./node_modules/.bin/vitest run src/app/error.test.tsx src/app/map/error.test.tsx
# Test Files  2 passed (2)
# Tests       11 passed (11)

Closes #143

[snowrugar-beep]

The Soroban contracts CI job is failing \xe2\x9d\x97\xe2\x83\xa3 \xe2\x80\x94 could you check the workflow logs and push a fix? Once green, ready to merge!