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

lib/src/core/insertr.js

@@ -104,10 +104,36 @@ export class InsertrCore {
     
     // Get element metadata
     getElementMetadata(element) {
+        const existingId = element.getAttribute('data-content-id');
+        
+        if (existingId) {
+            // Enhanced site - use existing ID
+            return {
+                contentId: existingId,
+                contentType: element.getAttribute('data-content-type') || this.detectContentType(element),
+                element: element,
+                hasExistingId: true
+            };
+        } else {
+            // Non-enhanced site - prepare context for backend ID generation
+            return {
+                contentId: null,  // Backend will generate
+                contentType: this.detectContentType(element),
+                element: element,
+                elementContext: this.extractElementContext(element),
+                hasExistingId: false
+            };
+        }
+    }
+    
+    // Extract element context for backend ID generation
+    extractElementContext(element) {
         return {
-            contentId: element.getAttribute('data-content-id') || this.generateDeterministicId(element),
-            contentType: element.getAttribute('data-content-type') || this.detectContentType(element),
-            element: element
+            tag: element.tagName.toLowerCase(),
+            classes: Array.from(element.classList),
+            original_content: element.textContent.trim(),
+            parent_context: this.getSemanticContext(element),
+            purpose: this.getPurpose(element)
         };
     }