feat: exhaustive e2e integration tests and bug fixes

[naasanov]

This PR adds comprehensive integration test coverage across all user roles and flows, and fixes several bugs discovered during the process of writing those tests.

Changes

Bug fixes:

• Fix dual-role access so students who are promoted to staff can still use student-facing flows
• Fix address field font size on party registration form to match surrounding inputs
• Add screen reader description to edit party dialog (accessibility)
• Change "Inactive" active hold filter label to "None" to match the displayed column value
• Fix student info chip "Completed Party Smart" field — narrow last_registered check to current school year only
• Fix column header stale filter/sort value caching issue in ColumnHeader
• Fix DatePicker silently snapping to today's date when an invalid/partial date string is entered — now leaves the field as-is until a valid date is parsed

E2e tests added:

• Auth flows: police login, signup, email flows, guards & error states
• Police: admin dashboard, officer dashboard, officer incidents, officer proximity
• Staff: accounts management, cross-tab chains, export, forms validation, info chips, invite & dual-role, restrictions (existing suites also updated)
• Student: dashboard tracker, first-time onboarding, gating & holds, info pages, profile
• Grouped sort and filter tests into dedicated describe blocks instead of a single flat group
• Added new helpers: mailpit.helpers.ts, seed-state.helpers.ts, xlsx.helpers.ts
• Added e2e/README.md documenting the test suite structure

[github-actions]

Test Results Summary

870 tests  +4   870 ✅ +4   1m 13s ⏱️ +8s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit f1e18a6. ± Comparison against base commit a5ee70c.

This pull request removes 3 and adds 7 tests. Note that renamed tests count towards both.

test.modules.account.account_router_test.TestAccountRouter ‑ test_create_invite_conflict_existing_account
test.modules.auth.auth_router_test.TestAuthRouter ‑ test_exchange_admin_no_invite_returns_403
test.modules.auth.auth_router_test.TestAuthRouter ‑ test_exchange_staff_no_invite_returns_403

test.modules.account.account_router_test.TestAccountRouter ‑ test_create_invite_conflict_existing_staff_or_admin
test.modules.account.account_router_test.TestAccountRouter ‑ test_create_invite_student_email_succeeds
test.modules.auth.auth_router_test.TestAuthRouter ‑ test_exchange_admin_no_invite_returns_student_tokens
test.modules.auth.auth_router_test.TestAuthRouter ‑ test_exchange_existing_student_promoted_via_staff_saml_with_invite
test.modules.auth.auth_router_test.TestAuthRouter ‑ test_exchange_staff_no_invite_returns_student_tokens
test.modules.auth.auth_router_test.TestAuthRouter ‑ test_exchange_staff_via_student_saml_creates_student_entity
test.modules.auth.auth_router_test.TestAuthRouter ‑ test_exchange_student_with_expired_staff_invite_succeeds