[PAP-3180] Move workspace switcher into sidebar (#4981)

## Thinking Path

> - Paperclip is the control plane for autonomous AI companies.
> - The board UI needs a clear persistent way to move between company
workspaces.
> - The previous layout kept company switching in a separate left rail,
which made the sidebar feel split between workspace selection and
navigation.
> - The workspace switcher belongs in the sidebar header so navigation
and workspace context stay together.
> - This pull request removes the separate company rail from the layout
and turns the sidebar company menu into the primary workspace switcher.
> - The benefit is a cleaner sidebar structure that keeps workspace
identity, switching, company actions, and navigation in one place.

## What Changed

- Removed the standalone `CompanyRail` from the main layout.
- Added the company/workspace switcher to the default, company settings,
and instance settings sidebars.
- Expanded `SidebarCompanyMenu` to list active workspaces, indicate the
current workspace, navigate out of instance settings when switching, and
expose add-company onboarding.
- Updated focused component tests for the new workspace-switcher
behavior.

## Verification

- `pnpm --filter @paperclipai/ui exec vitest run
src/components/SidebarCompanyMenu.test.tsx
src/components/CompanySettingsSidebar.test.tsx`
- `pnpm --filter @paperclipai/ui typecheck`
- `git diff --check`
- Visual smoke attempted against the managed dev server at
`http://127.0.0.1:57385`; a fresh browser context reached the
authenticated sign-in screen, so I could not capture an authenticated
sidebar screenshot from this heartbeat.

## Risks

- Low-to-medium UI risk: this changes the primary sidebar structure and
workspace-switching entry point.
- The instance-settings switch behavior now routes back to the selected
company dashboard when a workspace is selected.
- No migrations, API contracts, or lockfile changes.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent, tool-enabled, medium reasoning mode.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/components/SidebarCompanyMenu.test.tsx

@@ -14,7 +14,11 @@ const mockAuthApi = vi.hoisted(() => ({
   updateProfile: vi.fn(),
   signOut: vi.fn(),
 }));
+const mockNavigate = vi.hoisted(() => vi.fn());
+const mockOpenOnboarding = vi.hoisted(() => vi.fn());
+const mockSetSelectedCompanyId = vi.hoisted(() => vi.fn());
 const mockSetSidebarOpen = vi.hoisted(() => vi.fn());
+const mockLocation = vi.hoisted(() => ({ pathname: "/PAP/dashboard" }));
 
 vi.mock("@/api/auth", () => ({
   authApi: mockAuthApi,
@@ -24,18 +28,51 @@ vi.mock("@/lib/router", () => ({
   Link: ({ children, to, ...props }: { children: React.ReactNode; to: string }) => (
     <a href={to} {...props}>{children}</a>
   ),
+  useLocation: () => mockLocation,
+  useNavigate: () => mockNavigate,
 }));
 
 vi.mock("@/context/CompanyContext", () => ({
   useCompany: () => ({
+    companies: [
+      {
+        id: "company-1",
+        issuePrefix: "PAP",
+        name: "Acme Labs",
+        brandColor: "#3366ff",
+        status: "active",
+      },
+      {
+        id: "company-2",
+        issuePrefix: "STR",
+        name: "Strata",
+        brandColor: "#36a269",
+        status: "active",
+      },
+    ],
     selectedCompany: {
       id: "company-1",
+      issuePrefix: "PAP",
       name: "Acme Labs",
       brandColor: "#3366ff",
+      status: "active",
     },
+    setSelectedCompanyId: mockSetSelectedCompanyId,
   }),
 }));
 
+vi.mock("@/context/DialogContext", () => ({
+  useDialogActions: () => ({
+    openOnboarding: mockOpenOnboarding,
+  }),
+}));
+
+vi.mock("./CompanyPatternIcon", () => ({
+  CompanyPatternIcon: ({ companyName }: { companyName: string }) => (
+    <span aria-hidden="true">{companyName.slice(0, 1)}</span>
+  ),
+}));
+
 vi.mock("../context/SidebarContext", () => ({
   useSidebar: () => ({
     isMobile: false,
@@ -68,6 +105,7 @@ describe("SidebarCompanyMenu", () => {
       },
     });
     mockAuthApi.signOut.mockResolvedValue(undefined);
+    mockLocation.pathname = "/PAP/dashboard";
   });
 
   afterEach(() => {
@@ -94,7 +132,7 @@ describe("SidebarCompanyMenu", () => {
 
     expect(container.textContent).toContain("Acme Labs");
 
-    const trigger = container.querySelector('button[aria-label="Open Acme Labs menu"]');
+    const trigger = container.querySelector('button[aria-label="Open Acme Labs workspace switcher"]');
     expect(trigger).not.toBeNull();
 
     await act(async () => {
@@ -103,6 +141,9 @@ describe("SidebarCompanyMenu", () => {
     });
     await flushReact();
 
+    expect(document.body.textContent).toContain("Switch workspace");
+    expect(document.body.textContent).toContain("Strata");
+    expect(document.body.textContent).toContain("Add company...");
     expect(document.body.textContent).toContain("Invite people to Acme Labs");
     expect(document.body.textContent).toContain("Company settings");
     expect(document.body.textContent).toContain("Sign out");
@@ -122,4 +163,47 @@ describe("SidebarCompanyMenu", () => {
       root.unmount();
     });
   });
+
+  it("navigates to the selected workspace dashboard from company-prefixed routes", async () => {
+    mockLocation.pathname = "/PAP/issues";
+    const root = createRoot(container);
+    const queryClient = new QueryClient({
+      defaultOptions: { queries: { retry: false } },
+    });
+
+    await act(async () => {
+      root.render(
+        <QueryClientProvider client={queryClient}>
+          <SidebarCompanyMenu />
+        </QueryClientProvider>,
+      );
+    });
+    await flushReact();
+    await flushReact();
+
+    const trigger = container.querySelector('button[aria-label="Open Acme Labs workspace switcher"]');
+    expect(trigger).not.toBeNull();
+
+    await act(async () => {
+      trigger?.dispatchEvent(new PointerEvent("pointerdown", { bubbles: true, button: 0 }));
+      trigger?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    const strataItem = Array.from(document.body.querySelectorAll('[data-slot="dropdown-menu-item"]'))
+      .find((element) => element.textContent?.includes("Strata"));
+    expect(strataItem).toBeTruthy();
+
+    await act(async () => {
+      strataItem?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    expect(mockSetSelectedCompanyId).toHaveBeenCalledWith("company-2");
+    expect(mockNavigate).toHaveBeenCalledWith("/STR/dashboard");
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
 });