feat(calendar): add event notifications with alert evaluation

Add client-side calendar event notification system that evaluates JMAP
CalendarEventAlert triggers and displays toast notifications when alert
times are reached. Includes configurable notification sound, acknowledged
alert persistence, and proactive event fetching for background alerts.
Also mounts ToastContainer globally to fix silent toast failures.

ROADMAP.md

@@ -143,6 +143,9 @@ This document tracks the development status and planned features for JMAP Webmai
 - [x] i18n support with ICU pluralization (all 8 languages)
 - [x] Drag-and-drop event rescheduling (week/day time snap, month date move)
 - [x] iCalendar (.ics) file import via CalendarEvent/parse with preview and bulk create
+- [x] Event notifications with client-side alert evaluation and toast display
+- [x] Notification sound, acknowledged alert persistence (localStorage), proactive 24h event fetch
+- [x] Configurable notification settings (enable/disable, sound toggle)
 
 ### Email Filters
 - [x] JMAP Sieve Scripts (RFC 9661) with capability detection
@@ -173,6 +176,8 @@ This document tracks the development status and planned features for JMAP Webmai
 - [x] JMAP client method tests (identity: 20, contacts: 41)
 - [x] Unit tests for Sieve generator (50 tests)
 - [x] Unit tests for Sieve parser (14 tests)
+- [x] Unit tests for calendar alerts (36 tests)
+- [x] Unit tests for calendar notification store (8 tests)
 - [x] XSS attack vector testing
 - [x] Playwright E2E framework setup
 
@@ -188,7 +193,6 @@ This document tracks the development status and planned features for JMAP Webmai
 - [ ] Participant scheduling with iTIP invitations
 - [ ] Free/busy queries (Principal/getAvailability)
 - [ ] Calendar sharing UI (JMAP Sharing RFC 9670)
-- [ ] Calendar event notifications display
 - [ ] Email templates
 - [ ] Email encryption (PGP/GPG)
 - [ ] OAuth2/OIDC authentication (opt-in, Basic Auth remains default)