feat(web): complete beta signup flow and launch assets

[ntatschner]

What changed

• replace the gated signup dead-end with a working waitlist form
• keep login usable while offering the gated waitlist signup
• preserve invite tokens through signup errors
• regenerate launch screenshots from synthetic fixtures with visible demo disclosure
• record delivery and screenshot-scrub evidence

Why

The existing auth banner linked back to /, but the beta dismissal cookie prevented the full-screen gate and its form from reopening.

Validation

• full web unit suite: 800 passed
• focused auth and beta-gate browser flows: 12 passed
• focused waitlist/signup unit tests: 23 passed
• TypeScript type-check passed
• synthetic screenshot generator: 4 passed, visually reviewed