feat: show clearer analysis error messages for API failures

[vvatelot]

Summary

• Map POST errors 400, 401 and 403 (including excluded hosts) to dedicated French messages in the popup
• Clarify GET analysis failures 500, 520 and 521 so users know when the page is unreachable, not HTML, or not returning HTTP 200
• Stop treating a failed POST JSON body as a task id, and keep the technical API payload in the expandable error detail

Test plan

• Trigger a POST 400 with an unreachable URL and check the popup title/detail
• Trigger a POST 401 on a login-protected page
• Trigger a POST 403 on an antibot-protected page, and on an excluded host if possible
• Trigger analysis failures 500 / 520 (non-HTML resource) / 521 (non-200 page) via GET task polling
• Confirm a successful analysis still displays the result as before

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