Allow custom page selection beyond alternating order

[jeisaacs]

Summary

Introduce flexible page selection so users can define arbitrary sequencing rules instead of only alternating pages.

Why

• Some workflows require repeating pages (e.g., 2:1 ratios) or skipping blanks
• Unlocks mixed-mode outputs like grouping several pages from one file before switching
• Makes Dupl3x viable for more archival and booklet prep scenarios

Proposal

• Extend the processing schema to accept a declarative page map (drag-and-drop list, template presets, etc.)
• Validate selections in the UI and surface estimated output length
• Update the PDF service to follow the custom sequence safely

Acceptance Criteria

• Users can configure custom page mappings without editing config files
• Backend enforces rules (bounds checking, odd/even helpers) before processing
• Activity log records the mapping used
• Default alternating behavior remains unchanged when no custom mapping is provided