refactor: Flatten directory structure

Move project files from uzdb/ subdirectory to root directory for cleaner project structure.

Changes:
- Move frontend/ to root
- Move internal/ to root
- Move build/ to root
- Move all config files (go.mod, wails.json, etc.) to root
- Remove redundant uzdb/ subdirectory nesting

Project structure is now:
├── frontend/        # React application
├── internal/        # Go backend
├── build/           # Wails build assets
├── doc/             # Design documentation
├── main.go          # Entry point
└── ...

🤖 Generated with Qoder

internal/database/sqlite.go

@@ -0,0 +1,128 @@
+package database
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"go.uber.org/zap"
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+
+	"uzdb/internal/config"
+	"uzdb/internal/models"
+)
+
+var (
+	sqliteDB *gorm.DB
+	sqliteMu sync.RWMutex
+)
+
+// InitSQLite initializes the SQLite database for application data
+func InitSQLite(dbPath string, cfg *config.DatabaseConfig) (*gorm.DB, error) {
+	sqliteMu.Lock()
+	defer sqliteMu.Unlock()
+
+	// Ensure directory exists
+	dir := filepath.Dir(dbPath)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return nil, fmt.Errorf("failed to create database directory: %w", err)
+	}
+
+	// Configure GORM logger
+	var gormLogger logger.Interface
+	if config.Get().IsDevelopment() {
+		gormLogger = logger.Default.LogMode(logger.Info)
+	} else {
+		gormLogger = logger.Default.LogMode(logger.Silent)
+	}
+
+	// Open SQLite connection
+	db, err := gorm.Open(sqlite.Open(dbPath), &gorm.Config{
+		Logger: gormLogger,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("failed to open SQLite database: %w", err)
+	}
+
+	// Set connection pool settings
+	sqlDB, err := db.DB()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get underlying DB: %w", err)
+	}
+
+	sqlDB.SetMaxOpenConns(cfg.MaxOpenConns)
+	sqlDB.SetMaxIdleConns(cfg.MaxIdleConns)
+	sqlDB.SetConnMaxLifetime(time.Duration(cfg.MaxLifetime) * time.Minute)
+
+	// Enable WAL mode for better concurrency
+	if err := db.Exec("PRAGMA journal_mode=WAL").Error; err != nil {
+		config.GetLogger().Warn("failed to enable WAL mode", zap.Error(err))
+	}
+
+	// Run migrations
+	if err := runMigrations(db); err != nil {
+		return nil, fmt.Errorf("migration failed: %w", err)
+	}
+
+	sqliteDB = db
+
+	config.GetLogger().Info("SQLite database initialized",
+		zap.String("path", dbPath),
+		zap.Int("max_open_conns", cfg.MaxOpenConns),
+		zap.Int("max_idle_conns", cfg.MaxIdleConns),
+	)
+
+	return db, nil
+}
+
+// runMigrations runs database migrations
+func runMigrations(db *gorm.DB) error {
+	migrations := []interface{}{
+		&models.UserConnection{},
+		&models.QueryHistory{},
+		&models.SavedQuery{},
+	}
+
+	for _, model := range migrations {
+		if err := db.AutoMigrate(model); err != nil {
+			return fmt.Errorf("failed to migrate %T: %w", model, err)
+		}
+	}
+
+	config.GetLogger().Info("database migrations completed")
+	return nil
+}
+
+// GetSQLiteDB returns the SQLite database instance
+func GetSQLiteDB() *gorm.DB {
+	sqliteMu.RLock()
+	defer sqliteMu.RUnlock()
+	return sqliteDB
+}
+
+// CloseSQLite closes the SQLite database connection
+func CloseSQLite() error {
+	sqliteMu.Lock()
+	defer sqliteMu.Unlock()
+
+	if sqliteDB == nil {
+		return nil
+	}
+
+	sqlDB, err := sqliteDB.DB()
+	if err != nil {
+		return err
+	}
+
+	if err := sqlDB.Close(); err != nil {
+		return fmt.Errorf("failed to close SQLite database: %w", err)
+	}
+
+	sqliteDB = nil
+	config.GetLogger().Info("SQLite database closed")
+	return nil
+}