Milestone 5: Read-only frontend dashboard

[NkToGo]

Summary

Implements Milestone 5 as a read-only frontend dashboard.

Current progress:

• Slice 1: API and routing foundation
• Slice 2: Dataset and Validation Run dashboard
• Slice 3: Validation Run details and Issue filters
• Slice 4: Documentation, accessibility, and final verification

Slice 1

• Added durable frontend routing for / and /runs/:runId
• Added a Vite development proxy for relative /api requests
• Added handwritten TypeScript API contracts and runtime decoders
• Added a native-fetch API client with safe error handling
• Added abortable asynchronous resource handling
• Added focused API, routing, and request-lifecycle tests
• Added react-router-dom as the only new production dependency

Scope boundaries

• Read-only frontend only
• No backend or database changes
• No Flyway migration changes
• No backend CORS configuration
• No Compose or CI changes
• No write workflows
• No pagination or server-side filtering

Verification

• Focused frontend tests: 34 passed
• ESLint: passed
• Prettier check: passed
• TypeScript and Vite build: passed
• git diff --check: passed