Make the workspace right pane a first-class tab host (#3287)

* feat(app): add persistent workspace side panes

Keep the common review and file-browsing workflow beside the active agent without tearing down pane contents. Workspace tabs now share one compact chip language, assistant file links route into the retained right pane, and diff/file trees share stable geometry.

* fix(app): localize pull request empty state

* test(app): update explorer pane coverage

* fix(app): preserve explorer pane workflows

* test(app): make explorer tab selection idempotent

* fix(app): persist workspace review pins

* fix(app): preserve sidebar scrim drag hook

* fix(app): restore companion pane interactions

* test(app): follow selected sidebar surface

packages/app/e2e/support/helpers/file-explorer.ts

@@ -1,4 +1,5 @@
 import { expect, type Page } from "@playwright/test";
+import { openFilesPanel } from "./workspace-tabs";
 
 function fileExplorerTree(page: Page) {
   return page.getByTestId("file-explorer-tree-scroll");
@@ -9,8 +10,7 @@ function fileExplorerEntry(page: Page, name: string) {
 }
 
 export async function openFileExplorer(page: Page): Promise<void> {
-  await page.getByRole("button", { name: "Open explorer" }).first().click();
-  await page.getByTestId("explorer-tab-files").click();
+  await openFilesPanel(page);
   await expect(fileExplorerTree(page)).toBeVisible({ timeout: 30_000 });
 }