[GrantFox][Medium] Add deterministic notification preference handling

Summary

Add deterministic notification preference handling

Why this matters

Users need predictable control over settlement, market, and wallet notifications.

Scope

Persist scoped preferences, handle account changes safely, and reconcile server/client state.

Acceptance criteria

• Preferences are isolated by account.
• Defaults are explicit.
• Offline changes reconcile without duplication.
• Tests cover account switch, conflict, and reset.

Validation

Add regression coverage for existing behavior, failure modes, authorization boundaries, and compatibility. The implementation must pass the repository CI checks.

Non-goals

• Typo-only, formatting-only, or documentation-only changes.
• Unrelated refactors or dependency upgrades.
• Weakening existing security, authorization, CI, or production safeguards.

Contributor application

Before implementation, comment with relevant experience, a 1–4 bullet approach, and an estimate for opening the first draft PR. Wait for maintainer assignment before coding.

PR requirements

Use a feature branch, include Closes #<issue-number>, check every acceptance criterion, link criteria to code/tests, explain security and failure-mode considerations, and pass CI.

Reward-readiness

This is a substantive GrantFox campaign issue. Merge and CI success do not by themselves guarantee reward eligibility; final reward-readiness is determined by campaign review.

Start by locating the existing notification preference, account-switch, and server/client synchronization logic in the frontend. Add regression coverage for account switching, conflicts, resets, offline reconciliation, defaults, and authorization boundaries; done means each acceptance criterion is covered and the repository CI checks pass.