Transfer: add background job execution, runtime progress sink, LLM de…

[DeepThinkingZhouLiu]

…bug logging, and frontend job polling monitor

[DeepThinkingZhouLiu]

This PR stabilizes the MinerU-based template transfer flow and removes a few misleading failure/success paths.

Summary:

• disable the unfinished layout-check flow instead of auto-resuming with empty screenshots
• validate MinerU startup/upload inputs earlier (missing token, non-PDF upload)
• fix TeX \input / \include resolution so nested relative paths are resolved from the current file directory
• reorder the MinerU pipeline to copy assets before LLM transfer
• normalize copied MinerU image paths and rewrite Markdown image references to the final target filenames
• update the transfer UI so it no longer implies VLM layout review is active

Validation:

• frontend build passed with npm run build
• backend module import checks passed
• verified nested \input resolution with a targeted script
• verified duplicate-image rename + Markdown rewrite with a targeted script
• ran a real MinerU parsing flow with a valid MinerU token
• ran a full real transfer flow with a real OpenAI-compatible endpoint and confirmed the generated project compiled successfully

Known limitation:

• layout quality is still not fully solved; the flow now prioritizes correctness and honest status reporting over unfinished VLM-based layout validation