fix(top-miners): rename 'Leaderboard' tab to 'OSS Contributions' (#941)

[plind-junior]

Summary

Renames the first timeline tab on /top-miners from "Leaderboard" → "OSS Contributions" so it's parallel to the sibling "Discoveries" tab and accurately describes the tab's content.

 const TIMELINE_LABELS: Record<TimelineTab, string> = {
-  oss: 'Leaderboard',
+  oss: 'OSS Contributions',
   discoveries: 'Discoveries',
 };

That's the entire change — one line in src/pages/TopMinersPage.tsx.

Why

• The page itself is the Leaderboard — using "Leaderboard" as one of its sub-tabs is redundant and misleading.
• The sibling tab is Discoveries (issue-discovery rankings). Renaming this one to OSS Contributions makes the two tabs symmetric and self-describing: OSS Contributions vs. Discoveries.
• Internally the variant is already keyed as 'oss' and the watchlist UI already uses the "OSS" label for the same dimension — this aligns the user-facing tab with that vocabulary.

Related Issues

Fixes #941

Type of Change

• Bug fix
• Refactor (label only — no behavior change)
• New feature
• Documentation
• Other

Out of scope (per #941)

• No URL param change — ?timeline=oss stays as the default and existing bookmarks keep working.
• No SEO/page title change — the page still uses "Leaderboard" as its <title>; that describes the page, not the tab.
• No route or sidebar nav change — /top-miners and the sidebar entry are untouched.

Screenshots

Test plan

• Visit /top-miners — first tab reads "OSS Contributions" and is selected by default.
• Visit /top-miners?timeline=discoveries — second tab reads "Discoveries" and is selected.
• Click between the two tabs — URL updates correctly (?timeline=discoveries ↔ no param), table content swaps as before.
• Existing bookmarks with ?timeline=oss and ?timeline=discoveries still resolve to the correct tab.
• Browser tab title still says "Leaderboard" (page-level SEO unchanged).

Checklist

• Uses predefined theme tokens (no hardcoded colors)
• Responsive/mobile checked — Tabs component already responsive; only the label string changed
• Tested against the test API
• npm run format and npm run lint:fix have been run
• npm run build passes
• npm test passes (75/75)
• Screenshots included for UI/visual changes

[plind-junior]

@anderdc @e35ventura It's ready for review