feat: use a hazard-stripe border on staging

[jaspermayone]

Replace the solid amber frame on Weave staging with a yellow and black diagonal hazard stripe.

• The frame is 8px wide, fixed, and uses repeating-linear-gradient(45deg, #facc15 0 12px, #111 12px 24px). A mask cuts out the center.
• The frame ignores the pointer, takes no space and does not animate.
• The STAGING tab is black bold text on yellow, at the top center.
• Every layout still renders the partial: application, sessions, admin and consent.
• Production never shows it. The request spec covers both cases.

Tests: 1051 examples, 0 failures.