Add assignment-notifier service and client-side assignment notification support

[coder13]

Motivation

• Introduce a lightweight notifier service that polls WCIFs, detects assignment changes, and generates push notification jobs so users can be informed of assignment updates.
• Provide a client-side integration to register push subscriptions and allow users to enable assignment notifications in the app settings.

Description

• Add a new service scaffold at services/assignment-notifier including TypeScript sources (src/), compiled output (dist/), tsconfig.json, a local JSON store (data/store.json), and a README.md describing configuration and next steps.
• Implement snapshot/diff logic (createAssignmentSnapshots, buildNotificationJobs), WCIF client (fetchWcif), store persistence (readStore/saveStore), and a placeholder push sender that logs deliveries in src/ and compiled dist/.
• Wire up client-side notification utilities in src/lib/notifications/assignmentNotifications.ts with subscription registration, backend sync, and periodic check bootstrap, plus a React hook useAssignmentNotifications and its index export.
• Integrate UI and runtime hooks by importing bootstrapAssignmentNotificationChecks in src/App.tsx to auto-bootstrap when permission is granted and adding notification controls to src/pages/Settings/index.tsx, and add yarn scripts notify:service and notify:service:build to package.json.

Testing

• No automated tests were added or executed as part of this change.

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Codex Task

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