fix(web): synchronize milestone achievements with database to prevent duplicates

[Novice47]

Description

This PR addresses the issue where milestone achievements are spammed across multiple devices for a single user.

Cause

Currently, Stats.jsx relies on localStorage to check if a milestone has already been logged. When a user logs in from a new device (or clears browser data), the localStorage key is missing, causing isMilestoneAchieved to return false. This triggers duplicate requests to the backend to log the milestone, which spams the feed.

Solution

• Added getUserMilestones(userId) helper in activityService.js to fetch all already logged milestones from the database.
• • Implemented syncMilestonesToLocalStorage(milestones) helper in Stats.jsx to synchronize the achievements fetched from the database to the local device's localStorage during initial load.
• • Integrated the database fetch and sync check within loadData() before checkMilestones() runs. This ensures localStorage is seeded with existing achievements, preventing duplicates on new devices.
    Acknowledge: Developed with assistance from Antigravity.

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