[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/InstanceSidebar.tsx

@@ -5,6 +5,7 @@ import { pluginsApi } from "@/api/plugins";
 import { queryKeys } from "@/lib/queryKeys";
 import { SIDEBAR_SCROLL_RESET_STATE } from "@/lib/navigation-scroll";
 import { SidebarNavItem } from "./SidebarNavItem";
+import { SidebarCompanyMenu } from "./SidebarCompanyMenu";
 
 export function InstanceSidebar() {
   const { data: plugins } = useQuery({
@@ -14,11 +15,12 @@ export function InstanceSidebar() {
 
   return (
     <aside className="w-60 h-full min-h-0 border-r border-border bg-background flex flex-col">
-      <div className="flex items-center gap-2 px-3 h-12 shrink-0">
-        <Settings className="h-4 w-4 text-muted-foreground shrink-0 ml-1" />
-        <span className="flex-1 text-sm font-bold text-foreground truncate">
-          Instance Settings
-        </span>
+      <div className="flex items-center gap-1 px-3 h-12 shrink-0">
+        <SidebarCompanyMenu />
+      </div>
+      <div className="flex items-center gap-2 px-5 pb-3 shrink-0">
+        <Settings className="h-4 w-4 text-muted-foreground shrink-0" />
+        <span className="flex-1 truncate text-sm font-bold text-foreground">Instance Settings</span>
       </div>
 
       <nav className="flex-1 min-h-0 overflow-y-auto scrollbar-auto-hide flex flex-col gap-4 px-3 py-2">