perf: optimize above-fold image loading [closes #332]

[zigzagdev]

Summary

• HeritageCard: add isPriority prop — first 3 cards get loading="eager" + fetchPriority="high"; rest stay loading="lazy"
• Add width={400} height={320} and decoding="async" to all card images to prevent CLS
• HeritageList + SearchResultsPage: pass isPriority={index < 3}

Test plan

• First 3 cards load eagerly (check Network tab — no loading=lazy on first 3 images)
• No layout shift when images load in
• Below-fold images still lazy-load
• TypeScript compiles without errors

🤖 Generated with Claude Code

[zigzagdev]

Dropping this PR — image optimization (#332) is being removed from scope. The 403 issue was caused by aggressive loading attrs (fetchPriority=high + loading=eager), and the API's thumbnail_url points to document URLs rather than direct images. Reverting to simple lazy loading only.