fix(canvas): 主题存储 - 修复非法 theme 值导致 /create 崩溃 (#155)

useThemeStore persist 无 merge 校验,坏 session/后端同步失败时可能将
非法 theme 值写入 localStorage,导致 canvasThemes[非法值] 为 undefined,
渲染时访问 theme.node 抛出 "Cannot read properties of undefined
(reading 'node')",/create 页面崩溃。

4 层防线(只加校验,不回退代码):
- useThemeStore persist merge 白名单校验,非法值回退 dark
- useThemeStore setTheme 写入前校验,防止同步路径绕过 merge
- normalizeConfigSnapshot 入口兜底 undefined/partial 快照
- ModelPicker 与 CanvasResourceMentionTextarea 消费端加 fallback

Closes #154

Co-authored-by: sikongyue <3155157671@qq.com>

web/src/components/canvas/canvas-resource-mention-textarea.tsx

@@ -45,7 +45,8 @@ export const CanvasResourceMentionTextarea = forwardRef<HTMLTextAreaElement, Pro
     { value, references, onChange, onSubmit, onKeyDown, className, containerClassName, style, highlightLabels = true, mentionMenuWidth = 256, sendOnEnter = true, onContentSizeChange, ...props },
     forwardedRef,
 ) {
-    const theme = canvasThemes[useThemeStore((state) => state.theme)];
+    const rawTheme = useThemeStore((state) => state.theme);
+    const theme = canvasThemes[rawTheme as keyof typeof canvasThemes] ?? canvasThemes.dark;
     const containerRef = useRef<HTMLDivElement | null>(null);
     const textareaRef = useRef<HTMLTextAreaElement | null>(null);
     const editorRef = useRef<HTMLDivElement | null>(null);