fix(razor): stop /groups without id from returning 500

[Seme30]

Part of #789.

Problem

Razor /groups with no id returns 500 (Model.Group.Name NRE on a detail-only page). Next.js lists the four seeded groups.

Current gaps

• Razor GET /groups → "Sorry there was an error accessing that page."
• Next.js GET /groups → table of Clinical Operations, Data Governance Council, Enterprise Analytics, Revenue Cycle Analytics (Type, Source).
• Razor /groups?id=2 (Clinical Operations) matches Next.js: email, Department, Demo, and tabs Details / Users In Group / Reports In Group / Run Analytics / Report Runs / Analytics.

Razor references

• web/Pages/Groups/Index.cshtml
• web/Pages/Groups/Index.cshtml.cs
• GET /api/groups

Definition of done

• Visiting /groups without an id does not 500.
• Either show a group list like Next.js, or redirect to a valid group / empty state.
• Group detail with id stays intact.