fix(members): keep notifications above search

[dmagidson]

Closes #1432

summary

• lower the members directory sticky search stacking layer below the app header
• keep the notification panel above the search field without changing global modal or navigation layering

verification

• make agent-frontend-lint
• make agent-frontend-typecheck
• make agent-frontend-test — 1,269 passed, 1 skipped
• verified the notification dialog is the top painted element at the search-field overlap in the local browser
• full make agent-ci attempted; the backend portion hit 9 unrelated local SQLite/path-space failures after 2,172 passes