feat(search): virtualise results with ItemsRepeater + WrapLayout (#679)

[jaybarden1]

Summary

• Replace ItemsControl + WrapPanel with ItemsRepeater + WrapLayout — only visible cards are realised, off-screen cards do not hold live containers
• Wire thumbnail deferred loading to ItemsRepeater.ElementPrepared / ElementClearing (replacing ContainerPrepared / ContainerClearing)
• Bump application version 0.27.0 → 0.28.0 (feature)
• Re-applies changes that were reverted after PR feat(search): virtualise results with ItemsRepeater + WrapLayout (#679) #680 merged

Closes #679

Test plan

• dotnet clean && dotnet build --no-restore — zero errors, zero warnings ✅
• dotnet test --no-build — all passing, zero new failures ✅
• Card layout (180 px wide wrapping rows) visually identical to previous layout
• Scroll through >100 results and confirm off-screen cards are not held in memory
• Thumbnails load when cards scroll into view; cancel fires when scrolled out

🤖 Generated with Claude Code

[github-actions]

Test results

2 522 tests  ±0   2 521 ✅ ±0   4m 5s ⏱️ +8s
   10 suites ±0       1 💤 ±0 
   10 files   ±0       0 ❌ ±0 

Results for commit 1f78046. ± Comparison against base commit 5e6d291.