feature: mobile push notifications for price alerts

[nanaf6203-bit]

Summary

Price alerts are email only.

Affected area

• Add Web Push for supported browsers; opt-in toggle in settings; service-worker handler in ServiceWorkerRegistration.

Acceptance criteria

• Implement the change described above.
• Add or update unit/e2e tests locking in the new behaviour.
• Update README / docs to reflect the new state.

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Filed automatically as part of the PropChain code-review sweep.