Add GitHub-style study consistency heatmap (fixes #1175)

[anushkagupta200615-jpg]

Related Issue

Closes #ISSUE_NUMBER

Summary

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Changes Made

• Bullet list of implemented changes.
• ...

Testing

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Screenshots

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Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)

Description

This PR resolves #1175 by adding a new GitHub-style Contribution Heatmap to visualize long-term user consistency.

Resolves: #1175

Changes Made

• Added a new Statistics tab to the sidebar navigation to house the new feature.
• Built a lightweight, Vanilla JS/CSS implementation of a 365-day Contribution Heatmap (renderHeatmap).
• Created a getStudyActivityByDate function to aggregate the user's completed tasks dynamically.
• Added interactive tooltips to each heatmap cell displaying the date and exact count of completed tasks.
• Included varying shading intensity CSS matching GitHub's color scheme based on task frequency.
• Wired the heatmap to the global store to instantly reflect completions without requiring a refresh.

Visuals

Users can now navigate to the Statistics section and view their consistency for the trailing year at a glance:

(Drag and drop the heatmap image here)