Add a fun design to our error pages

[Neophytis]

The current error pages (403, 404, 429, 500, 502, 503, maintenance) are plain placeholders. We need on-brand designs with BeWelcome colours, logo, a simple illustration, and a clear recovery action per page — written in a tone that feels friendly to members, not just technical. Self-contained HTML only (inline CSS/SVG, no external requests).

Current pages:

403	404

429	500

502	503

[thisismeonmounteverest]

403, 404 and 500 are clearly in the realm of the app and shouldn't be replaced.

How would a 503 be served while not maintance.html gets served?

[Neophytis]

@thisismeonmounteverest Traefik knows when an app is unhealthy. Indeed most of the 4xx should be handled by the app. 429 not. This ticket is about designing the pages and not where they rest so let me redraft it to not brin any ambiguity to the conversation. This is mostly about being a fun project for any designers willing to help. There is another ticket for technical design that got mixed up.