feat(user-mgmt): clarify group permissions combine via union (closes #1001)

[cristim]

Summary

• Adds a one-line helper note in the User Management group-assignment expand panel explaining that a user's effective permissions are the union of all their groups' permissions, so assigning a lower-privilege group alongside a higher-privilege one is redundant, not contradictory.
• No blocking or mutual-exclusion logic added -- working-as-designed per question(user-mgmt): should contradictory group combinations be blocked? #1001 Option A decision.
• Two unit tests added asserting the hint text is rendered in .expand-panel-groups after the expand button is clicked.

Changes

frontend/src/users/userList.ts (renderUserExpandPanel): one new <p class="text-muted"> line after the <h5>Group Membership</h5> heading, using only the HTML entity ’ (right-single-quote) -- static text, no dynamic data injection.

frontend/src/__tests__/users.test.ts: new describe('group union permission hint (issue #1001)') block with two tests.

Test plan

• npm run build -- clean (pre-existing entrypoint-size warning only)
• npx tsc --noEmit -- no errors
• npm test -- --testPathPattern=users.test -- 206/206 pass, including the two new tests
• Open a user row in the User Management UI and verify the hint appears below the "Group Membership" heading

closes #1001

[cristim]

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ℹ️ Review info

⚙️ Run configuration

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Run ID: aa23b7a2-ba90-4fc1-b743-a31e45178718

📥 Commits

Reviewing files that changed from the base of the PR and between 3405060 and 5cb13d9.

📒 Files selected for processing (2)

• frontend/src/__tests__/users.test.ts
• frontend/src/users/userList.ts

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch feat/1001-group-union-hint

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