style: refine leaderboard history filters with clear controls

[marktech0813]

Summary

Makes the Resolved PR History filter/search controls recoverable: clear the search field, reset all filters in one click, and show a contextual empty state when a filter returns no matches.

Contribution Type

• Leaderboard UI

Motivation

Once a reviewer types a search or picks a non-All filter that yields zero rows, the archive looks empty with no obvious way back. The prior empty copy always said "No Resolved PRs", which is misleading when the archive has entries that simply don't match the active controls.

Changes

• Search field gains an inline clear (X) control when a query is present.
• Active filter/search shows a Clear chip in the filter row and an aria-live match summary.
• Empty state distinguishes "No Matching PRs" (filters active) from "No Resolved PRs", with a Clear filters action.
• Styles only on the approved Leaderboard surface (LeaderboardBoard.tsx, styles.css).

Testing

• Manual testing
• UI lint and build pass
• CI passes

Visual Evidence

Notes

Replaces Generation UI attempts (#121/#122) which the competition correctly rejected. Only touches approved Leaderboard UI files. One open Leaderboard UI PR for this contributor.

Made with Cursor

[marktech0813]

Closing in favor of reopening #122 with the same Leaderboard UI fix (competition allows only one open Leaderboard UI PR per contributor). Generation UI work on #122 was replaced with the approved Leaderboard surface.