[Frontend] Unify notification metadata types across context and list components

## Problem

NotificationList expects a discriminated metadata union while the context exposes an unconstrained record, causing a compile failure and unsafe rendering.

## Relevant code

`src/components/Notifications/NotificationList.tsx, src/types/notification.ts`

## Proposed scope

Create one canonical notification model, validate metadata by type, and test unknown metadata without crashing.

## Acceptance criteria

- [ ] The reported behavior is reproduced or characterized with a focused automated test before the fix.
- [ ] The implementation satisfies the proposed scope without weakening TypeScript, lint, authorization, privacy, or error handling.
- [ ] Success, empty/loading where applicable, failure, and boundary cases are covered.
- [ ] Existing related tests pass and new regression coverage is included.
- [ ] User-facing behavior remains accessible by keyboard and at mobile viewport sizes where UI is affected.
- [ ] Documentation is updated when the change alters configuration, contracts, security assumptions, or contributor workflow.

## Contributor notes

Base the work on the latest `main` branch. Keep unrelated refactors out of the pull request and include screenshots or recordings for visible changes. Never use real pet, medical, contact, wallet, or credential data in fixtures.

Start by reading src/components/Notifications/NotificationList.tsx and src/types/notification.ts to trace the metadata types between the context and list components. Add a focused automated test that characterizes the compile or rendering failure, including unknown metadata, then verify related states and tests pass without unsafe rendering.

Written by the indexing model from the issue text.