fix(desktop): don't crash when Prometheus discovery runs without a kubeconfig (#1787)

Fixes #1782

## What
Radar Desktop crashed the moment you opened Settings (or the Cost page)
on a machine with no usable kubeconfig — the exact state a fresh install
lands in.

## Why
Desktop deliberately keeps running when kubeconfig init fails so the UI
can show the error. But `RegisterCallbacks` still passed
`k8s.GetClient()` — a nil `*kubernetes.Clientset` — into
`prometheuspkg.Initialize` / `Reinitialize` /
`traffic.ReinitializeWithConfig`, all of which take
`kubernetes.Interface`. A typed nil in an interface is non-nil, so the
`k8sClient == nil` guard in `discover()` never fired.
`/api/opencost/summary` → `EnsureConnected` → `prom.Discover` then
dereferenced it inside a `wg.Go` worker, where chi's Recoverer can't
catch the panic, and the process exited.

## Fix
- `k8s.GetClientInterface()` returns an untyped nil when there is no
client; used at the three bootstrap sites that store the client in an
interface.
- Regression tests pin the untyped-nil and the pass-through case.

## Verified
- Reproduced on macOS with `make desktop-binary` + empty `$HOME`: click
Settings → `panic: nil pointer dereference` in
`pkg/prom.collectWellKnownLocations`, process gone.
- With the fix, the same build answers `/api/opencost/summary` with
`{"available":false,"reason":"query_error",…}` and `POST
/api/prometheus/connect` with `no Kubernetes client available for
discovery`; the process stays up.
- `go test ./internal/k8s ./internal/prometheus ./internal/app` and
`pkg/prom` green.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Small, targeted change at client handoff sites with tests; no auth or
data-path logic changes beyond preventing nil dereference when the
cluster client is missing.
> 
> **Overview**
> Fixes a **Radar Desktop process exit** when kubeconfig init fails but
the UI still runs (e.g. fresh install with no kubeconfig). Opening
Settings or hitting cost/Prometheus APIs could trigger discovery with a
**typed nil** `*kubernetes.Clientset` stored in `kubernetes.Interface`,
so downstream `client == nil` checks never ran and discovery panicked.
> 
> Adds **`k8s.GetClientInterface()`**, which returns an **untyped nil**
when there is no clientset, and switches Prometheus/traffic bootstrap
and reinit callbacks in `RegisterCallbacks` to use it instead of
`GetClient()`. **Regression tests** lock in untyped-nil vs pass-through
behavior.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
6edb345. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: nadaverell

internal/app/bootstrap.go

@@ -281,7 +281,7 @@ func RegisterCallbacks(cfg AppConfig, timelineStoreCfg timeline.StoreConfig) {
 	})
 
 	// Initialize Prometheus metrics client (must come before SetManualURL)
-	prometheuspkg.Initialize(k8s.GetClient(), k8s.GetConfig(), k8s.GetContextName())
+	prometheuspkg.Initialize(k8s.GetClientInterface(), k8s.GetConfig(), k8s.GetContextName())
 
 	if cfg.PrometheusURL != "" {
 		u, err := url.Parse(cfg.PrometheusURL)
@@ -319,14 +319,14 @@ func RegisterCallbacks(cfg AppConfig, timelineStoreCfg timeline.StoreConfig) {
 	}
 
 	k8s.RegisterTrafficFuncs(traffic.Reset, func() error {
-		return traffic.ReinitializeWithConfig(k8s.GetClient(), k8s.GetConfig(), k8s.GetContextName())
+		return traffic.ReinitializeWithConfig(k8s.GetClientInterface(), k8s.GetConfig(), k8s.GetContextName())
 	})
 
 	// Reinitialize carries the current manual URL + headers forward (including any
 	// applied live via /integrations/prometheus). Re-applying the captured startup
 	// cfg here would revert a live change on context switch, so we don't.
 	k8s.RegisterPrometheusFuncs(prometheuspkg.Reset, func() error {
-		prometheuspkg.Reinitialize(k8s.GetClient(), k8s.GetConfig(), k8s.GetContextName())
+		prometheuspkg.Reinitialize(k8s.GetClientInterface(), k8s.GetConfig(), k8s.GetContextName())
 		return nil
 	})
 	// Reinitialize only builds the new client; discovery for the new cluster has

internal/k8s/client.go

@@ -732,6 +732,19 @@ func GetClient() *kubernetes.Clientset {
 	return k8sClient
 }
 
+// GetClientInterface returns the clientset as kubernetes.Interface, or an
+// untyped nil when no client exists. Callers that store the result in an
+// interface must use this rather than GetClient(): a nil *Clientset wrapped in
+// an interface is non-nil, so "client == nil" guards downstream never fire and
+// the first API call panics.
+func GetClientInterface() kubernetes.Interface {
+	c := GetClient()
+	if c == nil {
+		return nil
+	}
+	return c
+}
+
 func GetClientSafetySnapshot() (*kubernetes.Clientset, string) {
 	clientMu.RLock()
 	defer clientMu.RUnlock()

internal/k8s/client_interface_test.go

@@ -0,0 +1,26 @@
+package k8s
+
+import (
+	"testing"
+
+	"k8s.io/client-go/kubernetes"
+)
+
+func TestGetClientInterfaceIsUntypedNilWithoutClient(t *testing.T) {
+	prev := SetTestClient(nil)
+	t.Cleanup(func() { SetTestClient(prev) })
+
+	if got := GetClientInterface(); got != nil {
+		t.Fatalf("GetClientInterface() = %#v, want untyped nil", got)
+	}
+}
+
+func TestGetClientInterfaceReturnsClient(t *testing.T) {
+	c := &kubernetes.Clientset{}
+	prev := SetTestClient(c)
+	t.Cleanup(func() { SetTestClient(prev) })
+
+	if got := GetClientInterface(); got != kubernetes.Interface(c) {
+		t.Fatalf("GetClientInterface() = %#v, want the configured clientset", got)
+	}
+}