fix(web): 本地优先保存与 design 页体验修复

- 保存先写本地缓存，登录后可选云端同步；退出登录不再因文件操作跳转登录

- 修复 Layui 遮罩残留、design2 保存后线条、toast 被 finally 提前关闭

- 保存过程恢复 loading spinner；成功提示 2.5 秒

- 云端 payload 瘦身与体积超限提示；后端 schema 迁移与 FileService 容错

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend-web/js/design2/core.js

@@ -552,10 +552,6 @@ function relayoutDesign2Canvas() {
     if (typeof window.soonDesign2ScaleBgSources === 'function') {
         window.soonDesign2ScaleBgSources();
     }
-    if (window.is_bgi_add) {
-        canvas1.requestRenderAll();
-        canvas2.requestRenderAll();
-    }
 }
 
 function soonDesign2ScaleBgSources() {
@@ -572,16 +568,6 @@ function soonDesign2ScaleBgSources() {
     }
 }
 
-function soonDesign2BgOverlayMetrics(imgEl) {
-    if (!imgEl || !imgEl.complete || imgEl.naturalWidth <= 0) return null;
-    var z = zoom || window.zoom || 0.5;
-    var w = parseFloat(imgEl.style.width);
-    if (!w || w <= 0) w = imgEl.naturalWidth * z;
-    var h = parseFloat(imgEl.style.height);
-    if (!h || h <= 0) h = imgEl.naturalHeight * z;
-    return { w: w, h: h };
-}
-
 function soonDesign2PrepareBgSources(done) {
     var sf = document.getElementById('source_front');
     var sb = document.getElementById('source_back');
@@ -634,7 +620,6 @@ function soonDesign2Kickoff() {
 
 window.soonDesign2ScaleBgSources = soonDesign2ScaleBgSources;
 window.soonDesign2PrepareBgSources = soonDesign2PrepareBgSources;
-window.soonDesign2BgOverlayMetrics = soonDesign2BgOverlayMetrics;
 window.soonDesign2Kickoff = soonDesign2Kickoff;
 
 initRulers();
@@ -2037,10 +2022,10 @@ function applyFontList(data) {
         $('#count_font_family').append("<option value='" + item + "'>" + item + '</option>')
     }
 }
-if (typeof ipcRenderer !== 'undefined' && ipcRenderer) {
-    ipcRenderer.on('font-list', function(event, data) { applyFontList(data); })
-} else if (typeof window !== 'undefined' && window.platformBridge && window.platformBridge.getSystemFonts) {
+if (typeof window !== 'undefined' && window.platformBridge && window.platformBridge.getSystemFonts) {
     window.platformBridge.getSystemFonts().then(applyFontList)
+} else if (typeof ipcRenderer !== 'undefined' && ipcRenderer) {
+    ipcRenderer.on('font-list', function(event, data) { applyFontList(data); })
 }
 function applySysLan(data) {
     let lang = localStorage.getItem('lang')