[Feature] Compare mode — side-by-side player stat comparison

[grovecj]

Summary

Allow users to select 2 or more players and compare their stats side-by-side.

Details

Backend

• GET /api/v1/compare?players={platform1}:{tag1},{platform2}:{tag2},...
• Fetches stats for all specified players (in parallel)
• Returns an array of player stat objects
• Max 4 players per comparison (to keep UI manageable)

Frontend — Compare Flow

1. Entry points:
   • "Compare" button on any player's stats page → adds them as first comparison target
   • /compare route with empty state prompting user to add players
   • Direct URL with query params: /compare?p=uno:player1,xbl:player2
2. Player selection:
   • Search bar to add players (reuse search component from [Feature] Player search — gamertag input and platform selection #9)
   • "Add Player" chips showing selected players with remove (X) button
   • Quick-add from squad members if a squad exists
3. Comparison table:
   • Rows = stats (K/D, Wins, Kills, etc.)
   • Columns = players
   • Highlight best value in each row (green bold) and worst (red subtle)
   • Percentage difference from the "best" shown in each cell

Frontend — Compare Visualizations

• Overlaid radar chart: All players on one radar chart with different colors
• Grouped bar chart: Side-by-side bars per stat per player
• Toggle between table view and chart view

Pinia Store (useCompareStore)

interface CompareStore {
  players: PlayerIdentifier[]  // { platform, gamertag }
  statsMap: Map<string, PlayerStats>
  addPlayer(platform: string, gamertag: string): void
  removePlayer(gamertag: string): void
  fetchAllStats(): Promise<void>
}

Acceptance Criteria

• Can add 2-4 players for comparison
• Side-by-side stat table with best/worst highlighting
• Overlaid radar chart with per-player colors
• Grouped bar chart comparison
• Players addable from search or squad quick-add
• Handles mixed results (one player private, others public)