test: add intent acceptance integration test

[Markadrian6399]

Summary

• Add src/app/solve/accept-intent.integration.test.tsx, wiring useAcceptIntent into the /solve open-intents feed end-to-end (real SolvePage, useOpenIntents, and useAcceptIntent — nothing mocked at the hook level)
• Mocks only the network boundary (@/lib/api), not the hooks under test
• Covers: open intent listed → accept clicked → success toast (via ToastViewport) → list revalidates and the intent disappears from the feed

Closes #76

Validation performed

• npx vitest run src/app/solve/accept-intent.integration.test.tsx (1 passed)
• npm test (full suite, 140 passed)
• npm run lint
• npx tsc --noEmit

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