feat(storefront): custom css themes

[OisinKyne]

feat(storefront): branding polish, docs + screenshots, inline-logo fix

Polish pass over the branding work: footer wording, documentation with real screenshots, and a genuine bug found while capturing them.

What's here

• "Powered by Obol" (not "Obol Stack") in the footers of the 402, landing, and job-status pages.
• Bug fix — inline logos were broken on Go-template pages: html/template rejects data: URIs in URL contexts (renders the literal #ZgotmplZ), so every logo inlined via sell info set --logo-file showed as a broken image on 402 and landing pages. Fixed with storefront.SafeAssetURL, which validates http(s)/data:image forms and returns template.URL; regression test TestPaymentRequiredHTML_InlineDataURILogo. Recorded as CLAUDE.md pitfall 21 (plus pitfall 20: template-exec errors silently fall back to JSON 402s).
• Docs with screenshots: docs/images/ gains four captures rendered through the real pipeline (402 renderer with fixture profiles; the actual Next.js storefront fed a fixture catalog; headless Chrome). README's branding section shows default-vs-branded storefront and 402 side by side with the full flag set; docs/guides/monetize-inference.md gains §1.8 "Brand Your Storefront"; CLAUDE.md documents all identity flags, the single-sanitizer rule, and the data-obol contract; the obol-stack-dev skill's rebuild matrix gains an internal/storefront/** row; the embedded sub-agent-business skill's branding nudge is updated.

Companion changes land separately in the gitbook (obol-stack/selling-services.md + assets) and skills (run-obol-stack) repos.

Testing

Full Go suite + storefront build green; the inline-logo regression test pins the template.URL bypass; screenshots double as a visual review of the light/dark/obol themes.

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