Add fastest lap highlight to leaderboard

[rawad-yared]

Summary

• Adds dynamic fastest lap tracking during race replay — the driver holding the fastest lap is highlighted with a purple background on the leaderboard, matching the official F1 broadcast style
• The fastest lap holder updates in real-time as faster laps are set throughout the race
• Computes lap completion times from FastF1 session data and walks through frames chronologically so the highlight appears at the correct moment

Changes

• src/f1_data.py — New _compute_fastest_lap_holders() function that embeds fastest_lap data (driver, formatted time, lap number) into each frame
• src/interfaces/race_replay.py — Passes fastest lap info from frame data to the window for leaderboard access
• src/ui_components.py — Renders purple highlight (background + text) for the fastest lap holder in LeaderboardComponent

Test plan

• Tested with 2025 Bahrain GP (Round 4) — fastest lap holder highlighted correctly
• Verify with races that have late fastest laps (e.g., drivers pitting for softs on final lap)
• Verify with sprint races
• Confirm cached data works after first --refresh-data run