Surface contact form submission errors instead of swallowing them

[devin-ai-integration]

Summary

The only JS in the repo is the contact form handler in index.html. Both failure paths (non-2xx Formspree response and a thrown fetch) ended in a bare errorMsg.hidden = false with the caught error discarded, so a validation rejection, a rate limit, and an offline browser were indistinguishable — for the visitor and in the console.

Now the error is propagated to both audiences:

• non-ok responses are parsed for Formspree's { errors: [{ message }] } / { error } payload, falling back to HTTP <status> <statusText>; JSON parse failure is itself logged rather than ignored
• the detail is appended to the existing Estonian error text (… (detail)) and every failure is console.error-ed
• missing #contactForm / #formSuccess / #formError now throw instead of failing with a TypeError on addEventListener, and the submit button is disabled for the duration of the request (re-enabled in finally) so a failed submit can't be double-fired.

Link to Devin session: https://app.devin.ai/sessions/74774155788442d78377a73aa25bec90
Requested by: @liikane

[devin-ai-integration]

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