fix(ui): Make read-only content selectable with a Copy context menu (#48)

Root cause: body is user-select: none and each feature had to opt back in; the Markdown preview (and other read-only views) never did, and the app-wide contextmenu guard left no Copy item.
Fix: shared selectableText class (src/ui) for read-only content, and a fallback Copy context menu from browserGuards when right-clicking a selection or a selectable area (component menus still win).
Siblings covered: Quick Documentation / hover / completion docs, hover errors, Quick Definition, debugger value tooltip, inspection messages, message dialogs (no drag on body text), notification titles, About dialog; existing selectable consoles now get Copy too.
Verified in-app on macOS: select in Markdown preview, right-click Copy puts the text on the clipboard; tab context menu unchanged.

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/ui/selectableText.test.ts

@@ -0,0 +1,93 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { copyMenuFor, copyText, selectableText } from "./selectableText";
+
+function mount(html: string) {
+  document.body.innerHTML = html;
+  return document.body;
+}
+
+function select(node: Node) {
+  const range = document.createRange();
+  range.selectNodeContents(node);
+  const sel = window.getSelection()!;
+  sel.removeAllRanges();
+  sel.addRange(range);
+}
+
+afterEach(() => {
+  window.getSelection()?.removeAllRanges();
+  document.body.innerHTML = "";
+  vi.restoreAllMocks();
+});
+
+describe("selectableText", () => {
+  it("is a real class name", () => {
+    expect(selectableText).toBeTruthy();
+  });
+});
+
+describe("copyMenuFor", () => {
+  it("offers Copy of the selection in a selectable pane (issue #36: Markdown preview)", () => {
+    mount(`<div class="${selectableText}" role="document"><p id="p">Hello <b id="b">world</b></p></div>`);
+    select(document.getElementById("p")!);
+    const items = copyMenuFor(document.getElementById("b"));
+    expect(items).toHaveLength(1);
+    expect(items![0]).toMatchObject({ label: "Copy", disabled: false });
+  });
+
+  it("offers a disabled Copy in a selectable pane with nothing selected", () => {
+    mount(`<div class="${selectableText}"><p id="p">Hello</p></div>`);
+    expect(copyMenuFor(document.getElementById("p"))![0]).toMatchObject({ label: "Copy", disabled: true });
+  });
+
+  it("resolves text-node targets", () => {
+    mount(`<div class="${selectableText}"><p id="p">Hello</p></div>`);
+    expect(copyMenuFor(document.getElementById("p")!.firstChild)).not.toBeNull();
+  });
+
+  it("offers Copy on a selection in any pane that opted in with its own CSS", () => {
+    mount(`<pre id="console">line 1</pre>`);
+    select(document.getElementById("console")!);
+    expect(copyMenuFor(document.getElementById("console"))![0]).toMatchObject({ disabled: false });
+  });
+
+  it("stays out of chrome without a selection", () => {
+    mount(`<button id="b">Run</button><div id="row">tree row</div>`);
+    expect(copyMenuFor(document.getElementById("b"))).toBeNull();
+    expect(copyMenuFor(document.getElementById("row"))).toBeNull();
+  });
+
+  it("ignores a selection elsewhere when right-clicking chrome", () => {
+    mount(`<div class="${selectableText}"><p id="p">Hello</p></div><div id="row">tree row</div>`);
+    select(document.getElementById("p")!);
+    expect(copyMenuFor(document.getElementById("row"))).toBeNull();
+  });
+
+  it.each(["<input id='t'>", "<textarea id='t'></textarea>", "<div id='t' contenteditable='true'>x</div>"])(
+    "leaves editable targets alone: %s",
+    (html) => {
+      mount(`<div class="${selectableText}">${html}</div>`);
+      expect(copyMenuFor(document.getElementById("t"))).toBeNull();
+    },
+  );
+
+  it("copies the text captured when the menu opened", async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    Object.defineProperty(navigator, "clipboard", { value: { writeText }, configurable: true });
+    mount(`<div class="${selectableText}"><p id="p">Hello</p></div>`);
+    select(document.getElementById("p")!);
+    const items = copyMenuFor(document.getElementById("p"))!;
+    window.getSelection()!.removeAllRanges();
+    (items[0] as { run: () => void }).run();
+    await Promise.resolve();
+    expect(writeText).toHaveBeenCalledWith("Hello");
+  });
+
+  it("falls back to execCommand when the clipboard API is missing", async () => {
+    Object.defineProperty(navigator, "clipboard", { value: undefined, configurable: true });
+    const exec = vi.fn().mockReturnValue(true);
+    document.execCommand = exec;
+    await copyText("x");
+    expect(exec).toHaveBeenCalledWith("copy");
+  });
+});