feat(sidecars): add proxied sidecar tabs (#279)

## Summary
- add SideCar support across the server and UI, including proxied tabs,
picker/settings flows, and websocket-aware proxying
- unify top-level tab handling so workspace instances and SideCars share
the same tab model and navigation flows
- limit SideCars to port-based services only, removing server-managed
process control from the final API and UI

---------

Co-authored-by: Shantur <shantur@Mac.home>
Co-authored-by: Shantur <shantur@Shanturs-MacBook-Pro-M5.local>

packages/ui/src/stores/instances.ts

@@ -36,6 +36,7 @@ import { clearCacheForInstance } from "../lib/global-cache"
 import { getLogger } from "../lib/logger"
 import { mergeInstanceMetadata, clearInstanceMetadata } from "./instance-metadata"
 import { showWorkspaceLaunchError } from "./launch-errors"
+import { activeSidecarToken } from "./sidecars"
 
 const log = getLogger("api")
 
@@ -109,6 +110,8 @@ function workspaceDescriptorToInstance(descriptor: WorkspaceDescriptor): Instanc
 }
 
 function ensureActiveInstanceSelected(): void {
+  if (activeSidecarToken()) return
+
   const current = activeInstanceId()
   const instanceMap = instances()
   if (current && instanceMap.has(current)) return