fix: resolve math formula display abnormal after editing report

This fix addresses the issue where math formulas become corrupted or
incorrectly displayed after editing the generated report in the editor.

**Root Cause:**
The issue occurred due to incompatibility between markdown processing
in the display component and the Tiptap editor:
1. Display component used \[\] and \(\) LaTeX delimiters
2. Tiptap Mathematics extension expects $ and 70868 delimiters
3. tiptap-markdown didn't have built-in math node serialization
4. Math syntax was lost/corrupted during editor save operations

**Solution Implemented:**
1. Created MathematicsWithMarkdown extension that adds markdown
   serialization support to Tiptap's Mathematics nodes
2. Added math delimiter normalization functions:
   - normalizeMathForEditor(): Converts LaTeX delimiters to $/70868
   - normalizeMathForDisplay(): Standardizes all delimiters to 70868
3. Updated Markdown component to use new normalization
4. Updated ReportEditor to normalize content before loading

**Changes:**
- web/src/components/editor/math-serializer.ts (new)
- web/src/components/editor/extensions.tsx
- web/src/components/editor/index.tsx
- web/src/components/deer-flow/markdown.tsx
- web/src/core/utils/markdown.ts
- web/tests/markdown-math-editor.test.ts (new)
- web/tests/markdown-katex.test.ts

**Testing:**
- Added 15 comprehensive tests for math normalization round-trip
- All tests passing (math editor + existing katex tests)
- Verified TypeScript compilation and linting

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

web/tests/markdown-katex.test.ts

@@ -3,7 +3,7 @@ import assert from "node:assert/strict";
 
 import katex from "katex";
 
-import { katexOptions } from "../src/core/markdown/katex";
+import { katexOptions } from "../src/core/markdown/katex.ts";
 
 function render(expression: string) {
   return katex.renderToString(expression, {