feat(ui): rebuild multi-document upload as extension of single-doc flow

[cpcloud]

Context

The multi-document upload feature (#818, commit 888e6eb) was a ground-up
reinvention that ignored the existing single-doc extraction flow. The result
was broken UX across the board: forced entity selection, broken file picker,
ugly staging list, no progress indicators, failed extractions with no
explanation.

Plan

1. Revert the batch doc feature entirely
2. Study the existing single-doc extraction overlay (which works well)
3. Rebuild multi-doc as a natural, incremental extension of the single-doc
   flow — one commit per component, each building on the last

supersedes #829
closes #489