ux(users): surface bulk-actions toolbar (delete + add-to-group)

[cristim]

Summary

• The Users tab checkboxes had no visible target: checking a row changed nothing visible from the user's perspective.
• Adds a #bulk-actions-bar toolbar to index.html that appears on first selection and hides when selection is cleared.
• Toolbar contains a selection counter, a Delete selected button, and an "Add to group..." select; all wired to the existing bulkDeleteUsers() / bulkAddToGroup() functions.
• Adds populateBulkGroupSelect() to userList.ts (called by loadUsers()) so the select stays in sync after every refresh.
• No bulk-change-role button added (removed in PR feat(auth): group-membership-only authorization, remove roles, require >=1 group (closes #907) #912).

closes #974

Test plan

• Open Settings > Users, verify toolbar is hidden with no rows checked
• Check one row: toolbar appears, count shows "1 users selected"
• Check two more rows: count updates to "3 users selected"
• Uncheck all rows: toolbar hides
• With rows selected, click "Delete selected", confirm dialog, verify rows are removed
• With rows selected, choose a group from "Add to group..." select, verify users are added and select resets to placeholder
• All 1940 tests pass (node_modules/.bin/jest --no-coverage)
• TypeScript: no new errors (tsc --noEmit)
• Build succeeds (webpack)

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[cristim]

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[cristim]

Rebased on feat/multicloud-web-frontend. Resolved conflict in frontend/src/users/handlers.ts (additive: kept base's removal of bulkChangeRole import, added populateBulkGroupSelect import from this PR).

[cristim]

pr-iterate: CR appears to be blocked by org-level usage limits; this PR is otherwise progressing (rebase complete, mergeStateStatus=UNSTABLE with CI pending). Re-invoke /pr-iterate 977 after the org's CR billing is topped up to get a substantive CR pass. To clear the billing block marker after billing is fixed: rm ~/.claude/agent-comms/cr-rate-limit-deadline.txt