Add Phase 2: Local SQLite database with sync functionality

Implements complete offline-first architecture with SQLite caching and MariaDB synchronization.

Key features:
- Local SQLite database for offline operation (data/quoteforge.db)
- Connection settings with encrypted credentials
- Component and pricelist caching with auto-sync
- Sync API endpoints (/api/sync/status, /components, /pricelists, /all)
- Real-time sync status indicator in UI with auto-refresh
- Offline mode detection middleware
- Migration tool for database initialization
- Setup wizard for initial configuration

New components:
- internal/localdb: SQLite repository layer (components, pricelists, sync)
- internal/services/sync: Synchronization service
- internal/handlers/sync: Sync API handlers
- internal/handlers/setup: Setup wizard handlers
- internal/middleware/offline: Offline detection
- cmd/migrate: Database migration tool

UI improvements:
- Setup page for database configuration
- Sync status indicator with online/offline detection
- Warning icons for pending synchronization
- Auto-refresh every 30 seconds

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/middleware/offline.go

@@ -0,0 +1,43 @@
+package middleware
+
+import (
+	"log/slog"
+
+	"github.com/gin-gonic/gin"
+	"git.mchus.pro/mchus/quoteforge/internal/localdb"
+	"gorm.io/gorm"
+)
+
+// OfflineDetector creates middleware that detects offline mode
+// Sets context values:
+//   - "is_offline" (bool) - true if MariaDB is unavailable
+//   - "localdb" (*localdb.LocalDB) - local database instance for fallback
+func OfflineDetector(mariaDB *gorm.DB, local *localdb.LocalDB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		isOffline := !checkMariaDBOnline(mariaDB)
+
+		// Set context values for handlers
+		c.Set("is_offline", isOffline)
+		c.Set("localdb", local)
+
+		if isOffline {
+			slog.Debug("offline mode detected - MariaDB unavailable")
+		}
+
+		c.Next()
+	}
+}
+
+// checkMariaDBOnline checks if MariaDB is accessible
+func checkMariaDBOnline(mariaDB *gorm.DB) bool {
+	sqlDB, err := mariaDB.DB()
+	if err != nil {
+		return false
+	}
+
+	if err := sqlDB.Ping(); err != nil {
+		return false
+	}
+
+	return true
+}