Highlight manifest body labels

[walker1211]

Summary

• Render card manifest labels as Markdown emphasis so video sections and news summary labels share the same visual treatment.
• Keep text fitting budgets aligned with the emphasized label prefixes.

Test plan

• go test ./internal/app -run 'Test(ServiceGeneratePreviewBuildsCardManifest|ServiceGeneratePreviewFitsCardManifest|ServiceGeneratePreviewReturnsErrorForUnknownCardManifestSectionField|ServiceGeneratePreviewBuildsNoImageCardManifestItemsAsTextCaption|ServiceGeneratePreviewBuildsCardManifestDeckWithoutAI|BuildCardManifestDeckWrapsElectronicPicklesCoverTitle|ServiceGeneratePreviewFitsCardManifestTextForNewsCards)'
• go test ./...
• go build -o ./mark2note ./cmd/mark2note