Open threads in place - the duplicate side panel is gone

The right-hand thread panel repeated exactly what the flat timeline already
showed. Opening a thread now focuses the center pane itself: back control,
board state, the root message with its replies, board intervention
controls, and a thread reply composer - one conversation surface, never
two. The release contract also catches up with the icon shipping inside
the package, a pin the icon commit should have updated.

tests/e2e/desktop.spec.ts

@@ -101,8 +101,13 @@ test("onboards, hires, and creates work in the Electron shell", async () => {
     await page.getByRole("button", { name: "Send" }).click();
     await expect(page.getByText("@Board").first()).toBeVisible();
     await expect(page.getByText("/escalate").first()).toBeVisible();
+    // Threads open in place: the center pane becomes the thread and the
+    // channel composer is gone until you go back.
     await page.getByRole("button", { name: "Open thread" }).first().click();
     await expect(page.getByRole("textbox", { name: "Thread reply" })).toBeVisible();
+    await expect(page.getByRole("textbox", { name: "Message" })).not.toBeVisible();
+    await page.getByRole("button", { name: "Close thread" }).click();
+    await expect(page.getByRole("textbox", { name: "Message" })).toBeVisible();
     await page.getByRole("button", { name: "Approvals" }).click();
     await expect(page.getByText("Escalate release").first()).toBeVisible();