[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/Layout.tsx

@@ -1,7 +1,6 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { useQuery } from "@tanstack/react-query";
 import { Outlet, useLocation, useNavigate, useNavigationType, useParams } from "@/lib/router";
-import { CompanyRail } from "./CompanyRail";
 import { Sidebar } from "./Sidebar";
 import { InstanceSidebar } from "./InstanceSidebar";
 import { CompanySettingsSidebar } from "./CompanySettingsSidebar";
@@ -336,7 +335,6 @@ export function Layout() {
             )}
           >
             <div className="flex flex-1 min-h-0 overflow-hidden">
-              <CompanyRail />
               {isInstanceSettingsRoute ? (
                 <InstanceSidebar />
               ) : isCompanySettingsRoute ? (
@@ -354,7 +352,6 @@ export function Layout() {
         ) : (
           <div className="flex h-full flex-col shrink-0">
             <div className="flex flex-1 min-h-0">
-              <CompanyRail />
               <div
                 className={cn(
                   "overflow-hidden transition-[width] duration-100 ease-out",