Issue/317: Add friendship ranking tab to assist credit history

[tanapl]

Summary

• Add new friendship ranking tab to assist credit history page
• Display user pairs ranked by total assist credits exchanged or
  transaction count
• Implement medal-based color theming for top 3 positions

Features

• Toggle sorting between total amount and transaction count
• Gold/silver/bronze styling for top 3 friendship pairs
• Responsive design with user icons and statistics

[Copilot]

Pull Request Overview

This PR introduces a new “Friendship Ranking” tab on the assist credit history page, showing user pairs ranked by total assist credits or transaction count with medal-based theming.

• Adds a new tab trigger and content pane for friendship rankings in the history route
• Implements FriendshipRanking component to fetch, aggregate, sort, and display top pairs
• Provides toggle buttons for sorting and medal-style theming for top 3 ranks

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkgs/frontend/app/routes/$treeId_.assistcredit.history.tsx	Imported FriendshipRanking, added “フレンドシップ” tab trigger and content pane
pkgs/frontend/app/components/assistcredit/FriendshipRanking.tsx	New component aggregating GraphQL data into friendship pairs, sorting, and rendering styled items