Branded 404 / 422 / 500 pages

What changed

When something goes wrong, you no longer see a Rails default error screen.
We now render branded 404, 422, and 500 pages with clear navigation and a
one-click link to contact support via Telegram or email.

Why it matters

• No more dead-ends — users always have a way forward
• Correct HTTP status codes, so Google indexes errors properly
• Support link is visible exactly when people need it