docs: add automations architecture page

[enyst]

Summary

• Adds a new /arch frontend page documenting the OpenHands Automations architecture, including system components, lifecycle flows, state transitions, data model, execution modes, injected environment, and design boundaries.
• Registers the static route before the dynamic automation detail route and adds an Architecture link from the automations list.
• Adds route coverage for the architecture page and navigation link.

Testing

• cd frontend && npm run test -- architecture automations-list
• cd frontend && npm run lint
• cd frontend && npm run build

Fixes #175.

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This PR was created by an AI agent (OpenHands) on behalf of enyst.

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