feat: implement collision-free lightweight hierarchical ID generation

- Replace content-hash based ID generation with position-based algorithm
- Use file + element identity + position index + hash for unique IDs
- Generate human-readable prefixes (e.g. index-lead-, index-p-2-)
- Add collision-resistant hash suffixes for guaranteed uniqueness
- Update Generate() to accept filePath parameter for context
- Fix ID collisions where hero and footer elements shared same ID
- Clean demo site files removing all data-content-id attributes
- Preserve insertr-gate elements for authentication functionality

Results: Hero gets 'index-lead-2-fc31f2', footer gets 'index-p-13-99fd13'
No more content cross-contamination between different elements.

internal/api/handlers.go

@@ -713,6 +713,7 @@ func (h *ContentHandler) generateContentID(ctx *ElementContext) string {
 	}
 
 	// Use existing parser ID generator
+	// For API-generated IDs, use a placeholder filePath since we don't have file context
 	idGenerator := parser.NewIDGenerator()
-	return idGenerator.Generate(virtualNode)
+	return idGenerator.Generate(virtualNode, "api-generated")
 }