feat: refactor Nakama plugin into generic multi-game match engine

### Highlights
- Introduced generic match engine (`generic_match.go`) implementing dynamic GameRules-based runtime.
- Added modular structure under `/plugins`:
  - /plugins/game      → GameRules interface + TicTacToe + Battleship rule sets
  - /plugins/structs   → Board, Player, MatchState generic structs
  - /plugins/modules   → matchmaking + RPC handlers + match engine
- Migrated TicTacToe logic into reusable rule implementation.
- Added Battleship game support using same engine.
- Updated matchmaking to accept { game, mode } for multi-game routing.
- Updated UI contract: clients must send `game` (and optional `mode`) when joining matchmaking.
- Removed hardcoded TicTacToe match registration.
- Registered a single “generic” authoritative match with ruleset registry.
- Normalized imports under local dev module path.
- Ensured MatchState and Board are now generic and reusable across games.
- Added strict requirement for `game` metadata in match flow (error if missing).
- Cleaned initial state creation into MatchInit with flexible board dimensions.
- Improved MatchLeave for proper forfeit handling through GameRules.

### Result
The server now supports an unlimited number of turn-based board games
via swappable rulesets while keeping a single authoritative Nakama match loop.

plugins/structs/player.go

@@ -0,0 +1,19 @@
+package game
+
+// Player represents a participant in the match.
+type Player struct {
+	UserID   string            `json:"user_id"`
+	Username string            `json:"username"`
+	Index    int               `json:"index"`
+	Metadata map[string]string `json:"metadata,omitempty"`
+}
+
+// NewPlayer creates a new player object.
+func NewPlayer(userID, username string, index int) *Player {
+	return &Player{
+		UserID:   userID,
+		Username: username,
+		Index:    index,
+		Metadata: make(map[string]string),
+	}
+}