When a username like 'christianlouis.de' (containing a dot) was submitted
on the signup page, FastAPI returned a 422 with detail as an array of
Pydantic validation error objects. The JS code assigned that array directly
to `this.error`, causing Alpine.js x-text to render '[object Object]'.
Two fixes applied in signup.html:
1. Client-side validation: check username length and pattern in submit()
before the API call, with clear human-readable error messages.
2. Server error handling: detect when data.detail is an Array and extract
each entry's .msg field, joining them into a readable string.
Also adds a regression test to confirm the 422 response format for an
invalid username (with dot) includes a list detail with msg fields.
Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
copilot-swe-agent[bot]
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