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/parser/parser.go

@@ -90,15 +90,15 @@ func (p *Parser) parseFile(filePath string) ([]Element, []string, error) {
 // findInsertrElements recursively finds all elements with "insertr" class
 func (p *Parser) findInsertrElements(node *html.Node, filePath string, elements *[]Element, warnings *[]string) {
 	if node.Type == html.ElementNode {
-		classes := getClasses(node)
+		classes := GetClasses(node)
 
 		// Check if element has "insertr" class
-		if containsClass(classes, "insertr") {
+		if ContainsClass(classes, "insertr") {
 			if isContainer(node) {
 				// Container element - expand to viable children
 				viableChildren := findViableChildren(node)
 				for _, child := range viableChildren {
-					childClasses := getClasses(child)
+					childClasses := GetClasses(child)
 					element, warning := p.createElement(child, filePath, childClasses)
 					*elements = append(*elements, element)
 					if warning != "" {
@@ -181,13 +181,13 @@ func (p *Parser) resolveContentID(node *html.Node) (string, bool) {
 // detectContentType determines the content type based on element and classes
 func (p *Parser) detectContentType(node *html.Node, classes []string) ContentType {
 	// Check for explicit type classes first
-	if containsClass(classes, "insertr-markdown") {
+	if ContainsClass(classes, "insertr-markdown") {
 		return ContentMarkdown
 	}
-	if containsClass(classes, "insertr-link") {
+	if ContainsClass(classes, "insertr-link") {
 		return ContentLink
 	}
-	if containsClass(classes, "insertr-text") {
+	if ContainsClass(classes, "insertr-text") {
 		return ContentText
 	}