chore(editor): address image-caption review (#221)

- docs: add CHANGELOG Unreleased/Added entry for editable image captions
- test: export sanitizeCaption and add vitest unit coverage
  (whitespace collapse, trim, 500-char boundary)
- refactor: drop duplicate .imageCaption CSS module class, keep the
  global .image-caption as the single source
- docs: fix turndown image-caption comment (video rule emits a markdown
  link, not a <div>)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/editor/components/common/use-caption-control.test.ts

@@ -0,0 +1,56 @@
+import { describe, it, expect } from "vitest";
+import { sanitizeCaption } from "@/features/editor/components/common/use-caption-control.tsx";
+
+/**
+ * `sanitizeCaption` = collapse every whitespace run to a single space + trim +
+ * cap at 500 chars. Captions are plain visible text, so this is a softer
+ * normalization than alt-text sanitization.
+ */
+describe("sanitizeCaption", () => {
+  it("trims leading and trailing whitespace", () => {
+    expect(sanitizeCaption("  hello  ")).toBe("hello");
+  });
+
+  it("collapses internal whitespace runs to a single space", () => {
+    expect(sanitizeCaption("a   b    c")).toBe("a b c");
+  });
+
+  it("treats tab, newline and CRLF as whitespace", () => {
+    expect(sanitizeCaption("a\tb")).toBe("a b");
+    expect(sanitizeCaption("a\nb")).toBe("a b");
+    expect(sanitizeCaption("a\r\nb")).toBe("a b");
+    expect(sanitizeCaption("line1\n\n\nline2")).toBe("line1 line2");
+  });
+
+  it("treats unicode whitespace (no-break space) as a separator", () => {
+    // U+00A0 NO-BREAK SPACE is matched by the \s class.
+    expect(sanitizeCaption("a b")).toBe("a b");
+  });
+
+  it("returns empty string for whitespace-only input", () => {
+    expect(sanitizeCaption("   ")).toBe("");
+    expect(sanitizeCaption("")).toBe("");
+  });
+
+  it("keeps a caption at the 500-char limit unchanged", () => {
+    const exact = "x".repeat(500);
+    expect(sanitizeCaption(exact)).toHaveLength(500);
+    expect(sanitizeCaption(exact)).toBe(exact);
+  });
+
+  it("slices a caption longer than 500 chars down to 500", () => {
+    const tooLong = "y".repeat(600);
+    const result = sanitizeCaption(tooLong);
+    expect(result).toHaveLength(500);
+    expect(result).toBe("y".repeat(500));
+  });
+
+  it("collapses whitespace before applying the 500-char cap", () => {
+    // 250 "a b" groups => "a b a b ..." which after collapse is 499 chars,
+    // adding a trailing pair pushes past 500 and gets sliced.
+    const input = "a  b ".repeat(120); // lots of double spaces
+    const result = sanitizeCaption(input);
+    expect(result.length).toBeLessThanOrEqual(500);
+    expect(result).not.toMatch(/\s{2,}/);
+  });
+});