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/browser/appearance-theme-picker.spec.ts

@@ -35,7 +35,7 @@ test("keeps the selected workspace visible in Pure black", async ({ page }, test
     await row.click();
 
     await expect(row).toHaveAttribute("aria-selected", "true");
-    await expect(row).toHaveCSS("background-color", "rgb(22, 22, 22)");
+    await expect(row).toHaveCSS("background-color", "rgb(17, 17, 17)");
     await page.screenshot({
       path: testInfo.outputPath("pure-black-selected-workspace.png"),
       fullPage: true,