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/localdb/models.go

@@ -0,0 +1,122 @@
+package localdb
+
+import (
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
+	"time"
+)
+
+// AppSetting stores application settings in local SQLite
+type AppSetting struct {
+	Key       string    `gorm:"primaryKey" json:"key"`
+	Value     string    `gorm:"not null" json:"value"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+func (AppSetting) TableName() string {
+	return "app_settings"
+}
+
+// LocalConfigItem represents an item in a configuration
+type LocalConfigItem struct {
+	LotName   string  `json:"lot_name"`
+	Quantity  int     `json:"quantity"`
+	UnitPrice float64 `json:"unit_price"`
+}
+
+// LocalConfigItems is a slice of LocalConfigItem that can be stored as JSON
+type LocalConfigItems []LocalConfigItem
+
+func (c LocalConfigItems) Value() (driver.Value, error) {
+	return json.Marshal(c)
+}
+
+func (c *LocalConfigItems) Scan(value interface{}) error {
+	if value == nil {
+		*c = make(LocalConfigItems, 0)
+		return nil
+	}
+	var bytes []byte
+	switch v := value.(type) {
+	case []byte:
+		bytes = v
+	case string:
+		bytes = []byte(v)
+	default:
+		return errors.New("type assertion failed for LocalConfigItems")
+	}
+	return json.Unmarshal(bytes, c)
+}
+
+func (c LocalConfigItems) Total() float64 {
+	var total float64
+	for _, item := range c {
+		total += item.UnitPrice * float64(item.Quantity)
+	}
+	return total
+}
+
+// LocalConfiguration stores configurations in local SQLite
+type LocalConfiguration struct {
+	ID             uint             `gorm:"primaryKey;autoIncrement" json:"id"`
+	UUID           string           `gorm:"uniqueIndex;not null" json:"uuid"`
+	ServerID       *uint            `json:"server_id"`        // ID on MariaDB server, NULL if local only
+	Name           string           `gorm:"not null" json:"name"`
+	Items          LocalConfigItems `gorm:"type:text" json:"items"` // JSON stored as text in SQLite
+	TotalPrice     *float64         `json:"total_price"`
+	CustomPrice    *float64         `json:"custom_price"`
+	Notes          string           `json:"notes"`
+	IsTemplate     bool             `gorm:"default:false" json:"is_template"`
+	ServerCount    int              `gorm:"default:1" json:"server_count"`
+	CreatedAt      time.Time        `json:"created_at"`
+	UpdatedAt      time.Time        `json:"updated_at"`
+	SyncedAt       *time.Time       `json:"synced_at"`
+	SyncStatus     string           `gorm:"default:'local'" json:"sync_status"` // 'local', 'synced', 'modified'
+	OriginalUserID uint             `json:"original_user_id"`                   // UserID from MariaDB for reference
+}
+
+func (LocalConfiguration) TableName() string {
+	return "local_configurations"
+}
+
+// LocalPricelist stores cached pricelists from server
+type LocalPricelist struct {
+	ID        uint       `gorm:"primaryKey;autoIncrement" json:"id"`
+	ServerID  uint       `gorm:"not null" json:"server_id"` // ID on MariaDB server
+	Version   string     `gorm:"uniqueIndex;not null" json:"version"`
+	Name      string     `json:"name"`
+	CreatedAt time.Time  `json:"created_at"`
+	SyncedAt  time.Time  `json:"synced_at"`
+	IsUsed    bool       `gorm:"default:false" json:"is_used"` // Used by any local configuration
+}
+
+func (LocalPricelist) TableName() string {
+	return "local_pricelists"
+}
+
+// LocalPricelistItem stores pricelist items
+type LocalPricelistItem struct {
+	ID          uint    `gorm:"primaryKey;autoIncrement" json:"id"`
+	PricelistID uint    `gorm:"not null;index" json:"pricelist_id"`
+	LotName     string  `gorm:"not null" json:"lot_name"`
+	Price       float64 `gorm:"not null" json:"price"`
+}
+
+func (LocalPricelistItem) TableName() string {
+	return "local_pricelist_items"
+}
+
+// LocalComponent stores cached components for offline search
+type LocalComponent struct {
+	LotName        string    `gorm:"primaryKey" json:"lot_name"`
+	LotDescription string    `json:"lot_description"`
+	Category       string    `json:"category"`
+	Model          string    `json:"model"`
+	CurrentPrice   *float64  `json:"current_price"`
+	SyncedAt       time.Time `json:"synced_at"`
+}
+
+func (LocalComponent) TableName() string {
+	return "local_components"
+}