[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/CompanySettingsSidebar.test.tsx

@@ -53,6 +53,10 @@ vi.mock("./SidebarNavItem", () => ({
   },
 }));
 
+vi.mock("./SidebarCompanyMenu", () => ({
+  SidebarCompanyMenu: () => <div>Workspace switcher</div>,
+}));
+
 vi.mock("@/api/sidebarBadges", () => ({
   sidebarBadgesApi: mockSidebarBadgesApi,
 }));