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/models/pricelist.go

@@ -0,0 +1,58 @@
+package models
+
+import (
+	"time"
+)
+
+// Pricelist represents a versioned snapshot of prices
+type Pricelist struct {
+	ID           uint       `gorm:"primaryKey" json:"id"`
+	Version      string     `gorm:"size:20;uniqueIndex;not null" json:"version"` // Format: YYYY-MM-DD-NNN
+	Notification string     `gorm:"size:500" json:"notification"`                // Notification shown in configurator
+	CreatedAt    time.Time  `json:"created_at"`
+	CreatedBy    string     `gorm:"size:100" json:"created_by"`
+	IsActive     bool       `gorm:"default:true" json:"is_active"`
+	UsageCount   int        `gorm:"default:0" json:"usage_count"`
+	ExpiresAt    *time.Time `json:"expires_at"`
+	ItemCount    int        `gorm:"-" json:"item_count,omitempty"` // Virtual field for display
+}
+
+func (Pricelist) TableName() string {
+	return "qt_pricelists"
+}
+
+// PricelistItem represents a single item in a pricelist
+type PricelistItem struct {
+	ID          uint    `gorm:"primaryKey" json:"id"`
+	PricelistID uint    `gorm:"not null;index:idx_pricelist_lot" json:"pricelist_id"`
+	LotName     string  `gorm:"size:255;not null;index:idx_pricelist_lot" json:"lot_name"`
+	Price       float64 `gorm:"type:decimal(12,2);not null" json:"price"`
+	PriceMethod string  `gorm:"size:20" json:"price_method"`
+
+	// Price calculation settings (snapshot from qt_lot_metadata)
+	PricePeriodDays  int      `gorm:"default:90" json:"price_period_days"`
+	PriceCoefficient float64  `gorm:"type:decimal(5,2);default:0" json:"price_coefficient"`
+	ManualPrice      *float64 `gorm:"type:decimal(12,2)" json:"manual_price,omitempty"`
+	MetaPrices       string   `gorm:"size:1000" json:"meta_prices,omitempty"`
+
+	// Virtual fields for display
+	LotDescription string `gorm:"-" json:"lot_description,omitempty"`
+	Category       string `gorm:"-" json:"category,omitempty"`
+}
+
+func (PricelistItem) TableName() string {
+	return "qt_pricelist_items"
+}
+
+// PricelistSummary is used for list views
+type PricelistSummary struct {
+	ID           uint       `json:"id"`
+	Version      string     `json:"version"`
+	Notification string     `json:"notification"`
+	CreatedAt    time.Time  `json:"created_at"`
+	CreatedBy    string     `json:"created_by"`
+	IsActive     bool       `json:"is_active"`
+	UsageCount   int        `json:"usage_count"`
+	ExpiresAt    *time.Time `json:"expires_at"`
+	ItemCount    int64      `json:"item_count"`
+}