fix(leaderboard): fix tabs filtering and refetching dynamic scopes

[yush-1018]

Changes:

• Client Routing: Migrated tabs in LeaderboardContent to be router-driven via Next.js useRouter and useSearchParams.
• Backend Scopes: Added 'monthly' scope (calculating XP earned in the last 30 days) and 'friends' scope (fetching followed developers on GitHub) to getLeaderboard.
• Organization Tab Mapping: Mapped the 'organization' tab to the 'cohort' scope, dynamically fetching the user's active/latest cohort from the database if not specified.
• Empty States: Added descriptive empty state cards for when a user isn't enrolled in a cohort or has no followed friends registered on the platform.

Fixes #389

[vercel]

@yush-1018 is attempting to deploy a commit to the codersogs-3057's projects Team on Vercel.

A member of the Team first needs to authorize it.

[jakharmonika364]

Solid feature, but the friends-tab GitHub fetch (following list) is duplicated between entries and currentUserRank, and the second one skips the cache entirely, so every Friends tab load hits GitHub's API live no matter what. Can you dedupe that into one call? Also a couple anys crept into page.tsx that weren't there before.