Replace custom canvas save/load with built-in save_load_adapter

- Remove custom canvas selector UI (HTML popup, CSS styles, toolbar button)
- Remove all custom canvas JS functions (toggleCanvasSelector, loadCanvas,
  saveCanvas, autoSaveCanvas, startAutoSave, newCanvas, deleteCanvas, etc.)
- Add createSaveLoadAdapter() implementing TradingView's save_load_adapter:
  - Chart layouts: localStorage index + backend API storage (no backend changes)
  - Study/Drawing/Chart templates: pure localStorage
- Enable load_last_chart and auto_save_delay in widget config
- Add DEV_MODE auth bypass for localhost development
- Update auth-config.js with production API URL and OAuth client ID
- Update README.md and README_CN.md feature descriptions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/auth-config.js

@@ -1,4 +1,4 @@
-window.API_CONFIG = { baseUrl: 'https://xxx.xxx.com' };
+window.API_CONFIG = { baseUrl: 'https://api-view.cathiefish.org' };
 
 /**
  * Google OAuth Configuration
@@ -9,7 +9,7 @@ window.API_CONFIG = { baseUrl: 'https://xxx.xxx.com' };
 const AUTH_CONFIG = {
     // Client ID obtained from GCP Console
     // Replace with your actual Client ID
-    clientId: 'xxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com',
+    clientId: '136939086552-bt28icv34nucmjna8hfndr6bopr5qcp4.apps.googleusercontent.com',
 
     // Authentication success callback
     onSuccess: (user) => {