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/config.go

@@ -0,0 +1,23 @@
+package modules
+
+type BoardConfig struct {
+    Rows int
+    Cols int
+}
+
+type GameConfiguration struct {
+    Players int
+    Board   BoardConfig
+}
+
+// Static configuration for all supported games.
+var GameConfig = map[string]GameConfiguration{
+    "tictactoe": {
+        Players: 2,
+        Board:   BoardConfig{Rows: 3, Cols: 3},
+    },
+    "battleship": {
+        Players: 2,
+        Board:   BoardConfig{Rows: 10, Cols: 10},
+    },
+}