Keep grounding choices when the other file loads

[ColtenOuO]

Sumarry

In the lobby's document grounding section, every JD or resume upload rebuilt all three snippet lists from scratch. Checking a few JD requirements and then uploading a resume silently unchecked them, and the same happened in reverse.

Sample

Upload and select checkboxes

After uploading the resume

Fix

Before a file is parsed, the current selection is read and only the groups that file replaces are dropped (requirements for a JD, skills and anchors for a resume), since those indexes point into a list that no longer exists. The rest is restored when the lists are re-rendered.

• web/document-grounding.js: add retainedSelection(selected, kind)
• web/app.js: extract checkedGrounding() (shared with the Start handler) and pass the retained selection to renderGroundingChoices

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Summary by cubic

Fixes the lobby's document grounding so uploading a resume no longer clears JD requirement checkboxes, and vice versa.

• Only the groups the new file replaces lose their selections; other checked groups survive the re-render.
• Extracts checkedGrounding() so the Start handler and file loading share the same selection logic.

^{Written for commit f328c7b. Summary will update on new commits.}

[jserv]

Rebase latest main branch and resolve conflicts.

[ColtenOuO]

Thanks for reviewing!

Addressed all feedback from the inline comments and resolved merge conflicts.

[jserv]

Thank @ColtenOuO for contributing!