fix: systematic element matching bug in enhancement pipeline

- Problem: Element ID collisions between similar elements (logo h1 vs hero h1)
  causing content to be injected into wrong elements
- Root cause: Enhancer used naive tag+class matching instead of parser's
  sophisticated semantic analysis for element identification

Systematic solution:
- Enhanced parser architecture with exported utilities (GetClasses, ContainsClass)
- Added FindElementInDocument() with content-based semantic matching
- Replaced naive findAndInjectNodes() with parser-based element matching
- Removed code duplication between parser and enhancer packages

Backend improvements:
- Moved ID generation to backend for single source of truth
- Added ElementContext struct for frontend-backend communication
- Updated API handlers to support context-based content ID generation

Frontend improvements:
- Enhanced getElementMetadata() to extract semantic context
- Updated save flow to handle both enhanced and non-enhanced elements
- Improved API client to use backend-generated content IDs

Result:
- Unique content IDs: navbar-logo-200530 vs hero-title-a1de7b
- Precise element matching using content validation
- Single source of truth for DOM utilities in parser package
- Eliminated 40+ lines of duplicate code while fixing core bug

internal/api/models.go

@@ -31,13 +31,23 @@ type ContentVersionsResponse struct {
 	Versions []ContentVersion `json:"versions"`
 }
 
+// Element context for backend ID generation
+type ElementContext struct {
+	Tag             string   `json:"tag"`
+	Classes         []string `json:"classes"`
+	OriginalContent string   `json:"original_content"`
+	ParentContext   string   `json:"parent_context"`
+	Purpose         string   `json:"purpose"`
+}
+
 // Request models
 type CreateContentRequest struct {
-	ID        string `json:"id"`
-	SiteID    string `json:"site_id,omitempty"`
-	Value     string `json:"value"`
-	Type      string `json:"type"`
-	CreatedBy string `json:"created_by,omitempty"`
+	ID             string          `json:"id,omitempty"`              // For enhanced sites
+	ElementContext *ElementContext `json:"element_context,omitempty"` // For non-enhanced sites
+	SiteID         string          `json:"site_id,omitempty"`
+	Value          string          `json:"value"`
+	Type           string          `json:"type"`
+	CreatedBy      string          `json:"created_by,omitempty"`
 }
 
 type UpdateContentRequest struct {