feat: show local pricelists in offline mode

**Problem:**
Pricelist page showed empty list in offline mode even though
local pricelists existed in SQLite cache.

**Solution:**
Modified PricelistHandler.List() to fallback to local pricelists:

1. Check if server list is empty (offline)
2. Load from localDB.GetLocalPricelists()
3. Convert LocalPricelist to summary format
4. Add "synced_from": "local" field
5. Add "offline": true flag

**Response format:**
```json
{
  "offline": true,
  "total": 4,
  "pricelists": [
    {
      "version": "2026-02-02-002",
      "created_by": "sync",
      "synced_from": "local",
      "is_active": true
    }
  ]
}
```

**Impact:**
- ✅ Local pricelists visible in offline mode
- ✅ UI can show cached pricelist versions
- ✅ Users can browse pricelists without connection
- ✅ Clear indication of local/remote source

Part of Phase 2.5: Full Offline Mode

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

internal/handlers/pricelist.go

@@ -29,6 +29,36 @@ func (h *PricelistHandler) List(c *gin.Context) {
 		return
 	}
 
+	// If offline (empty list), fallback to local pricelists
+	if total == 0 && h.localDB != nil {
+		localPLs, err := h.localDB.GetLocalPricelists()
+		if err == nil && len(localPLs) > 0 {
+			// Convert to PricelistSummary format
+			summaries := make([]map[string]interface{}, len(localPLs))
+			for i, lpl := range localPLs {
+				summaries[i] = map[string]interface{}{
+					"id":          lpl.ServerID,
+					"version":     lpl.Version,
+					"created_by":  "sync",
+					"item_count":  0, // Not tracked
+					"usage_count": 0, // Not tracked in local
+					"is_active":   true,
+					"created_at":  lpl.CreatedAt,
+					"synced_from": "local",
+				}
+			}
+
+			c.JSON(http.StatusOK, gin.H{
+				"pricelists": summaries,
+				"total":      len(summaries),
+				"page":       page,
+				"per_page":   perPage,
+				"offline":    true,
+			})
+			return
+		}
+	}
+
 	c.JSON(http.StatusOK, gin.H{
 		"pricelists": pricelists,
 		"total":      total,