Major codebase cleanup after .insertr-add functionality overhaul

Consolidates duplicate code and removes technical debt accumulated during rapid development. This cleanup improves maintainability while preserving all functionality.

Backend cleanup:
- Remove unused legacy function findViableChildrenLegacy()
- Consolidate duplicate SQL null string helper functions into shared utils
- Unify text extraction functions across utils, engine, and id_generator
- Consolidate duplicate attribute getter functions into single implementation

Frontend cleanup:
- Remove duplicate authentication methods (authenticateWithOAuth vs performOAuthFlow)
- Remove unused hasPermission() method from auth.js
- Centralize repetitive API endpoint construction in api-client.js
- Reduce excessive console logging while preserving important error logs

Impact: -144 lines of code, improved maintainability, no functionality changes
All tests pass and builds succeed

🤖 Generated with [Claude Code](https://claude.ai/code)

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

internal/engine/database_client.go

@@ -172,7 +172,7 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			ID:               contentID,
 			SiteID:           siteID,
 			HtmlContent:      htmlContent,
-			OriginalTemplate: toNullString(originalTemplate),
+			OriginalTemplate: ToNullString(originalTemplate),
 			LastEditedBy:     lastEditedBy,
 		})
 		if err != nil {
@@ -191,7 +191,7 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			ID:               contentID,
 			SiteID:           siteID,
 			HtmlContent:      htmlContent,
-			OriginalTemplate: toNullString(originalTemplate),
+			OriginalTemplate: ToNullString(originalTemplate),
 			LastEditedBy:     lastEditedBy,
 		})
 		if err != nil {
@@ -210,13 +210,6 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 	}
 }
 
-// Helper function to convert string to sql.NullString
-func toNullString(s string) sql.NullString {
-	if s == "" {
-		return sql.NullString{Valid: false}
-	}
-	return sql.NullString{String: s, Valid: true}
-}
 
 // GetCollection retrieves a collection container
 func (c *DatabaseClient) GetCollection(siteID, collectionID string) (*CollectionItem, error) {