[Review] fix: don't import broken agents

[govambam]

Recreated from writer#1278 for Macroscope review.

Original PR: writer#1278 by @UladzislauK-Writer
Status: closed (merged)

Recreated using squash merge commit - exact merged state preserved.

Original PR: writer#1278

[macroscopeapp]

Add structured error handling to /api/import in writer.serve.get_asgi_app and surface import failures in BuilderHeaderMoreDropdown.vue to prevent importing broken agents

Introduce structured JSON error responses with summary and optional details in /api/import, validate archives before replacement in writer.app_runner.AppRunner, and display server error summaries and traces in the builder import modal in BuilderHeaderMoreDropdown.vue.

📍Where to Start

Start with the /api/import handler writer.serve.get_asgi_app in serve.py, then review import validation in app_runner.py, and the UI error display in BuilderHeaderMoreDropdown.vue.

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Macroscope summarized da82279.