feat(app): float composer trackers as pills over the transcript (#3482)

The subagents track and the task list were stacked cards above the
composer. Each one pushed the composer further down the pane and painted
an opaque band the timeline stopped at, and both spent a full row on a
count that fits in a pill.

Both are now pills that open a panel — a popover on wide screens, a sheet
on compact ones, from the existing menu engine rather than a fourth
floating-panel implementation. The row frame is shared, so a subagent row
and a task row sit on the same rail whatever their contents.

The bar mounts inside the transcript's animated container and pins itself
to the bottom edge, so scrolled content passes under the pills instead of
stopping above them. It stays a child of that container rather than a
sibling floating over the composer: on Android a view outside its parent's
bounds paints but takes no touches, and every pill is pressable. The
container carries the same keyboard transform the composer does, so the
pills track the composer's top edge when the keyboard moves.

The tracker state moves out of the composer with it. None of it was
composer state; it lived there because that is where it used to render.

packages/app/e2e/browser/provider-subagents.real.spec.ts

@@ -133,6 +133,8 @@ test.describe("real provider subagent timelines", () => {
         await expect(
           page.getByTestId("assistant-message").filter({ hasText: "ROOT_DONE" }).last(),
         ).toBeVisible({ timeout: 60_000 });
+        // Opening the subagent's tab closed the panel with the parent's pane.
+        await openSubagentsTrack(page);
         const archiveFinished = page.getByTestId("subagents-track-archive-finished");
         await expect(archiveFinished).toBeVisible({ timeout: 30_000 });
         await archiveFinished.click();

packages/app/e2e/support/helpers/subagents.ts

@@ -128,8 +128,17 @@ export async function seedParentWithCrossWorkspaceSubagent(
   };
 }
 
+/**
+ * Opens the subagents panel, and leaves it open if it already is — the pill is a toggle, and a
+ * second click would close the thing the caller is about to assert on. The panel also closes on
+ * its own whenever a row navigates away, so a flow that comes back to the parent reopens it.
+ */
 export async function openSubagentsTrack(page: Page): Promise<void> {
-  await page.getByTestId("subagents-track-header").click();
+  const panel = page.getByTestId("subagents-track-header-panel");
+  if ((await panel.count()) === 0) {
+    await page.getByTestId("subagents-track-header").click();
+  }
+  await expect(panel).toBeVisible({ timeout: 30_000 });
 }
 
 export async function expectSubagentRowVisible(page: Page, childId: string): Promise<void> {