Improve manifest text cards and retry handling

[walker1211]

Summary

• Keep source metadata out of no-image text-card bodies while preserving it in the bottom CTA.
• Retry AI deck generation when the CLI returns INTERNAL_ERROR or stream error.
• Carry over the sage-mist theme palette update and refresh its render snapshot digest.

Test plan

• go test ./internal/app ./internal/ai ./internal/deck
• UPDATE_GOLDEN=1 go test ./internal/render
• go test ./...