feat(extra-sheet) : add inline cell editing

lib/rich-text.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 
 import {
+  extraRichTextToPlainText,
   applyRichTextStylePatch,
   normalizeExtraRichText,
   richTextContentSignature,
@@ -34,6 +35,14 @@ describe("rich text sanitization", () => {
 });
 
 describe("legacy Extra rich text conversion", () => {
+  it("converts stored markup into safe multiline text for cell editing", () => {
+    expect(extraRichTextToPlainText(
+      "<p><strong>First</strong><br>line</p><p>Second &amp; safe<script>bad()</script></p>",
+    )).toBe("First\nline\nSecond & safe");
+    expect(normalizeExtraRichText(extraRichTextToPlainText("<p>A<br>B</p>")))
+      .toBe("<p>A<br>B</p>");
+  });
+
   it("escapes plain text, preserves newlines, and converts phrase colors", () => {
     expect(normalizeExtraRichText("<unsafe>\nMASSIVE damage", [
       { text: "MASSIVE", color: "#ff0000" },