feat: add in-app notification system with real-time delivery

Introduces a full in-app notification system with three types (simple,
boolean with server-side callbacks, navigate), three scopes (user, trip,
admin), fan-out persistence per recipient, and real-time push via
WebSocket. Includes a notification bell in the navbar, dropdown, dedicated
/notifications page, and a dev-only admin tab for testing all notification
variants.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/package-lock.json

@@ -5941,9 +5941,9 @@
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-      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.23.tgz",
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+      "version": "4.18.1",
+      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.18.1.tgz",
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       "dev": true,
       "license": "MIT"
     },