Keep filers in place when checking or changing files, and return detours to where they started

[nonprofittechy]

Summary

A tester uploading a fee waiver on Fees was sent to the full "Check your documents" page and back, losing their fee waiver choice. Fixing that surfaced a family of detours where the app forgot why the filer was on a page. This PR keeps people where they are, or returns them to where they came from.

1. Fee waiver uploads are checked on Fees

The new copy is previewed in place with This copy looks right / Remove and upload a different file. The fee waiver choice stays selected, and Continue waits for the check (the server also refuses to continue with unchecked files).

2. One detour model in workflow.py

• continue_url / continue_step decide where a finished step goes: back to Review or the handoff list, or to the single step that fixes what the change broke, still carrying the marker. That's Organize when a document has no filing type, and Find your case when an existing case isn't found yet.
• Only a step's completion decides. Intermediate actions (adding a person, removing a file) keep the marker and stay on the screen.
• Only two markers, review and handoff. They're checked in one place (return_target / clean_return_to) and never echoed back unchecked.
• "Change files" on the preview keeps the origin through upload and preview.

Bugs fixed along the way:

• The handoff middleware rewrote every successful POST to the handoff list. So Add a person on People bounced back to the list, and Find your case skipped Confirm your case. The middleware now only keeps the draft in the URL.
• Confirm case dropped the marker when sending unchecked handoff documents to the preview.
• The "needs organizing" check treated an empty document type as unfinished. Organize saves none when the court offers no choices, so with the check centralized this would have looped Organize ↔ Review. It now checks only the filing type.

3. Files added on the checklist are checked there

Previously Review sent the filer to the preview page and back for them. Confirm/remove now live in a shared document-checks endpoint and script used by both Fees and the checklist. Any supporting file can be removed there; the main document is still changed on the upload step.

Behavior changes to note

• Switching between a new and an existing case from Review now goes Find your case (if needed) → Organize → Review, instead of restarting the flow. This skips the checklist, which is guidance, not a gate.
• payment and document_checklist are no longer accepted as return markers.

Not in this PR

• On a handoff detour, the "Back" and "Continue" labels don't say "Back to your list" the way they say "Back to review".
• The handoff "missing details" list (issues_for) still requires document_type_code for every document, which can't be satisfied for courts with no confidentiality choices.

Testing

• uv run pytest: 1330 passed. New: test_detours.py, test_document_checks.py, test_checklist_document_checks.py.
• npm run test:unit: 70 passed. New: document-checks.test.js, plus coverage for Fees keeping Continue disabled while a check is pending.
• New opt-in Chromium test, 9 scenarios: PAYMENT_FLOW_BROWSER_TESTS=1 uv run pytest -s efile/tests/test_payment_flow_browser.py. It needs the PDF.js vendor files (npm ci in efile_app).
  • Covers: the reported Fees flow, Axe, mobile width, remove/re-upload, reload before checking, a stale second tab, an unreadable PDF, Change files from Review, and checklist checks.
  • The stale-tab scenario can log a SQLite "table is locked" error from the test server. That's specific to SQLite and doesn't occur on PostgreSQL.

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