feat: add notification preferences section to user profile

[aminamos]

Summary:

Resolves #138.
Adds a notification subscription management section to the user account profile, displaying active feed subscriptions, frequency, and toggle controls.

Expected behavior:

Authenticated users can view and toggle their notification subscriptions directly from /account.

Testing tips:

1. Checkout feat/138-notifications-user-profile.
2. Run yarn test.
3. Visit /account and inspect the Notifications tab.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with yarn test to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

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