refactor(server): move shared game logic into games/ and introduce GameConfig system

- Moved common/game.go → plugins/games/rules.go
  - Contains GameRules interface, MovePayload, Player abstraction
  - Centralizes all reusable game rule contract logic

- Added plugins/games/config.go
  - Introduces GameConfiguration struct (players, board size, etc.)
  - Added global GameConfig and RulesRegistry maps
  - Each game (tictactoe, battleship, etc.) now registers its config + rules

- Updated generic_match.go to use:
  - GameConfig for board/players initialization
  - RulesRegistry for rule lookup during MatchInit
  - Removed hardcoded TicTacToe behavior
  - Clean error returns when game param missing or invalid

- Updated folder structure:
    /plugins/
        /games
            rules.go       (formerly common/game.go)
            config.go
            tictactoe.go
            battleship.go
        /structs
        /modules
        main.go

- Ensures GenericMatch pulls:
    m.GameName, m.Mode, m.Config, m.Rules
  directly from config/registry at MatchInit

- Removes old duplicated logic and simplifies how games are registered

plugins/games/rules.go

@@ -0,0 +1,27 @@
+package games
+
+import "localrepo/plugins/structs"
+
+// MovePayload is used for incoming move data from clients.
+type MovePayload struct {
+    Data map[string]interface{} `json:"data"`
+}
+
+type GameRules interface {
+    // Number of players needed to start.
+    MaxPlayers() int
+
+    // Assign symbols/colors/pieces at start.
+    AssignPlayerSymbols(players []*Player)
+
+    // Apply a move.
+    // Returns: (changed, gameOver, winnerIndex)
+    ApplyMove(state *structs.MatchState, playerIdx int, payload MovePayload) (bool, bool, int)
+
+    // If a player leaves, who wins?
+    // Return:
+    //   >=0 → winner index
+    //   -1 → draw
+    //   -2 → invalid
+    ForfeitWinner(state *structs.MatchState, leaverIndex int) int
+}