style(theme): retire the rainbow for a cool dark gradient

[ivorycrayon]

What

• Replace the pastel-rainbow palette with a cool blue→teal→periwinkle gradient across every hype surface: hub, privacy, rules, apply, guild-names, admin (each carried its own --rainbow palette)
• Cool the dark background, card borders and accent glows to match; the white <hype> mark is unchanged
• Recompose og-card.png (1200×630) in the cool gradient
• Bump the og-card cache-bust ?v=2026-06-24 → ?v=2026-06-24b on every og:image / twitter:image (incl. the tbc generator)
• tbc/ keeps its fel-green WoW theme — it never used the rainbow

Why

The social card was retoned cool; leaving the live pages on the rainbow would mismatch the shared preview. The Discord embed colour in server/app.py is deferred to a backend deploy pass.

Test plan

• hub / privacy / guild-names render in the cool palette on a preview server
• no warm/rainbow tokens remain (grep) except the deferred backend colour
• index.html + tbc/index.html rebuilt from source — both --check green
• og-card recomposed, exactly 1200×630

[ivorycrayon]

Cool-dark retheme — palette swap only, no functional change. Rainbow retired across hub/privacy/rules/apply/guild-names/admin; cool og-card recomposed (1200×630) + cache-bust bumped to ?v=2026-06-24b. Leak grep clean. Both --check guards green; previews look right. Squash-merging; deploy-pull ships it (~60s).