feat: add JSON output format

[unhappychoice]

Summary

Add --json flag to output stats in JSON format for scripting and integration with other tools.

Interface

steamfetch --json           # Pretty-printed JSON
steamfetch --json --compact # Minified JSON (single line)

Output Example

{
  "username": "unhappychoice",
  "account_created": "2010-01-15",
  "account_age_years": 16,
  "steam_level": 42,
  "games": {
    "total": 486,
    "unplayed": 123,
    "unplayed_percent": 25
  },
  "playtime": {
    "total_hours": 2847,
    "top_games": [
      { "name": "Borderlands 3", "hours": 478 },
      { "name": "Coin Push RPG", "hours": 377 }
    ]
  },
  "recently_played": [
    { "name": "Elden Ring", "hours": 20, "minutes": 0 }
  ],
  "achievements": {
    "total": 3241,
    "possible": 5892,
    "percent": 55,
    "perfect_games": 24,
    "rarest": {
      "name": "Impossible Task",
      "game": "Dark Souls III",
      "percent": 0.1
    }
  }
}

Implementation

• Add Serialize derive to SteamStats and related structs
• Add --json CLI flag
• Use serde_json for serialization (already a dependency via reqwest)

[linear]

E-774