Support XHS news card collections

[walker1211]

Summary

• Add XHS news-card collection support with optional ending pages and the text-caption variant.
• Keep image-caption pages tied to exactly one image and hydrate local images before preview rendering.
• Update XHS publish metadata/config options, render snapshots, and documentation for collection workflows.

Test plan

• go test ./internal/ai -run 'TestBuildDeckJSONUsesStrictPrompt|TestBuildDeckJSONPromptPreservesFullVisibleCodeBlocks|TestBuildDeckJSONPromptSplitsOversizedFencedCodeBlocks' -count=1
• go test ./internal/ai ./internal/deck ./internal/render ./internal/app -count=1
• go test ./... -count=1
• go build -o ./mark2note ./cmd/mark2note
• Generated a prepare-only XHS preview and verified it did not publish