fix(seo): replace SVG data URI placeholders with proper fallback handling

[BuyWhere]

Summary

Fixes product card images rendering as generic SVG gradient placeholders (data:image/svg+xml) instead of real product photos or clean text fallbacks.

Ref: BUY-62196

Root Cause

Product card images on SEO landing pages (/air-purifier-singapore etc.) and search results were all displaying as SVG data URI placeholders due to:

1. isUsableProductImage() treated SVG data URIs as valid images
2. withFallbackImage() actively generated SVG placeholders via buildCategoryImage()
3. ProductGridCard had a special rendering branch for SVG data URIs
4. buildSeoLandingSchema() included SVG data URIs in JSON-LD
5. Search page hasUsableProductImage() didn't reject data: protocol URLs

Changes (3 files, +5/-13 lines)

src/lib/seo-landing-pages.ts

• isUsableProductImage(): reject SVG data URIs
• withFallbackImage(): return null instead of generating SVG placeholder
• buildSeoLandingSchema(): exclude SVG data URIs from JSON-LD image field

src/components/seo/SeoLandingPage.tsx

• ProductGridCard: remove SVG-specific rendering branch

src/app/search/SearchResultsClient.tsx

• hasUsableProductImage(): reject data: protocol URLs

Behavior After Fix

Scenario	Before	After
SEO card, no real image	SVG gradient placeholder	Brand text on gradient bg
SEO card, real image	Real photo	Real photo (unchanged)
JSON-LD image	SVG data URI	Omitted
Search result, no real image	SVG overlay	Clean text fallback

[BuyWhere]

Fix complete — PR ready for merge

PR: #218
Status: Open, mergeable, clean CI state

What changed (3 files, +5/-13 lines)

1. src/lib/seo-landing-pages.ts — isUsableProductImage() now rejects SVG data URIs; withFallbackImage() returns null instead of generating SVG placeholders; buildSeoLandingSchema() excludes SVG data URIs from JSON-LD.

2. src/components/seo/SeoLandingPage.tsx — Removed SVG-specific rendering branch from ProductGridCard.

3. src/app/search/SearchResultsClient.tsx — hasUsableProductImage() now rejects data: protocol URLs.

Expected behavior after merge

• SEO pages with no real product image: brand/merchant text on gradient background (clean fallback)
• SEO pages with real product image: unchanged
• Search results with no real image: clean text fallback instead of SVG overlay
• JSON-LD structured data: no more placeholder SVGs in image field

Next step

Merge PR #218 to deploy to Railway production.