feat(ui): 全站统一首页墨黑中性风格，固定亮色并移除光暗切换

- 全站紫/青/粉写死色值与 CSS 变量改为墨黑中性色板
- 移除 aurora 光斑/星点装饰，登录弹窗改纯白卡片
- theme.js 固定亮色并清除历史 dark 选择，删除前台/后台/画布切换按钮
- 模型页空状态文案对齐「保存渠道即自动同步」

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/src/components/canvas/CanvasNodeView.vue

@@ -240,8 +240,8 @@ defineExpose({ startEdit, commitNote })
   overflow: hidden;
 }
 .cv-node.is-selected {
-  border-color: rgb(124 92 252 / 0.7);
-  box-shadow: var(--glass-shadow-lg), 0 0 0 3px rgb(124 92 252 / 0.25), 0 10px 30px var(--accent-glow);
+  border-color: rgb(24 24 27 / 0.7);
+  box-shadow: var(--glass-shadow-lg), 0 0 0 3px rgb(24 24 27 / 0.25), 0 10px 30px var(--accent-glow);
 }
 .cv-node:active {
   cursor: grabbing;
@@ -337,8 +337,8 @@ html.dark .cv-node--note {
   justify-content: center;
   gap: 10px;
   background:
-    radial-gradient(130% 130% at 15% 0%, rgb(124 92 252 / 0.14), transparent 55%),
-    radial-gradient(130% 130% at 85% 100%, rgb(34 211 238 / 0.12), transparent 55%),
+    radial-gradient(130% 130% at 15% 0%, rgb(24 24 27 / 0.14), transparent 55%),
+    radial-gradient(130% 130% at 85% 100%, rgb(24 24 27 / 0.12), transparent 55%),
     var(--surface);
 }
 .cv-node__gen::after {
@@ -371,7 +371,7 @@ html.dark .cv-node--note {
   width: 34px;
   height: 34px;
   border-radius: 50%;
-  border: 3px solid rgb(124 92 252 / 0.2);
+  border: 3px solid rgb(24 24 27 / 0.2);
   border-top-color: var(--m-orange);
   border-right-color: var(--m-orange-2);
   animation: cv-spin 0.9s linear infinite;
@@ -417,7 +417,7 @@ html.dark .cv-node--note {
 }
 .cv-port:hover::after, .cv-port:focus-visible::after, .cv-port.is-active::after {
   transform: scale(1.3);
-  box-shadow: 0 0 0 4px rgb(124 92 252 / 0.25), 0 2px 10px var(--accent-glow);
+  box-shadow: 0 0 0 4px rgb(24 24 27 / 0.25), 0 2px 10px var(--accent-glow);
 }
 .cv-port.is-active::after { background: var(--m-orange); }
 .cv-port:focus-visible { outline: 2px solid var(--m-orange); outline-offset: -2px; border-radius: 50%; }

frontend/src/components/canvas/CanvasStage.vue

@@ -811,7 +811,7 @@ defineExpose({
   stroke: rgb(95 108 248 / 0.85);
   stroke-width: 2.4;
   stroke-dasharray: 10 6;
-  filter: drop-shadow(0 0 5px rgb(124 92 252 / 0.3));
+  filter: drop-shadow(0 0 5px rgb(24 24 27 / 0.3));
 }
 .cv-edge--hot {
   stroke: var(--m-orange);
@@ -843,8 +843,8 @@ defineExpose({
 }
 .cv-marquee {
   position: absolute;
-  border: 2px dashed rgb(124 92 252 / 0.5);
-  background: rgb(124 92 252 / 0.08);
+  border: 2px dashed rgb(24 24 27 / 0.5);
+  background: rgb(24 24 27 / 0.08);
   pointer-events: none;
   border-radius: 4px;
 }