Summary

• Add deck.max_pages with a default of 12 and a supported range of 3-18.
• Pass the configured maximum page count into AI deck prompt generation and deck validation.
• Allow preview and from-deck rendering flows to accept configured page limits.

Test plan

• go test ./internal/config -run 'TestLoad(AppliesDefaultDeckConfig|KeepsExplicitDeckMaxPages|RejectsInvalidDeckMaxPages)' -count=1
• go test ./internal/ai -run 'TestBuildDeckJSONUsesConfigured(MaxPages|Command)' -count=1
• go test ./internal/app -run 'TestServiceGenerate(PreviewPassesMaxPagesToAIBuilder|PreviewAcceptsConfiguredMaxPages|FromDeckAcceptsConfiguredMaxPages)' -count=1
• go test ./... -count=1
• go build -o ./mark2note ./cmd/mark2note
• Local XHS metadata smoke with deck.max_pages: 18