Fix the blank VPN tab, and stop one page taking the console with it

On a machine with wireguard-tools installed but no tunnel up — core, which is
the machine the console opens on — the tunnel list was a nil slice, which is
null in JSON, and the page took its length. That throws, React unmounts the
whole tree, and what is left is a blank window with nothing to go on.

Two fixes, because either alone leaves the failure possible. The parse returns
an empty list rather than nil, with a test that pins the marshalled shape; and
the page treats a missing list as an empty one, so a node still running an older
build cannot blank the console either.

And the reason a small mistake had such a large blast radius: nothing caught it.
Each page now renders inside a boundary, so a fault costs that page and says
what it was, while the rail, the switcher and every other tab carry on. The
terminals are deliberately left outside it — they hold live WebSockets, and
remounting them to recover from somebody else's error would drop the sessions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/vpn/vpn.go

@@ -138,7 +138,9 @@ func Look(ctx context.Context) Report {
 //
 // The private key is in the second column and is read past without being kept.
 func parseDump(out string) []Tunnel {
-	var tunnels []Tunnel
+	// An empty list, never nil: this is JSON to a browser that will take its length, and a
+	// null there is a blank page rather than "no tunnels".
+	tunnels := []Tunnel{}
 	byName := map[string]int{}
 	sc := bufio.NewScanner(strings.NewReader(out))
 	sc.Buffer(make([]byte, 0, 64<<10), 1<<20)

internal/vpn/vpn_test.go

@@ -172,3 +172,22 @@ func TestNeverConnectedHasNoTimestamp(t *testing.T) {
 		t.Errorf("json = %s, want the handshake of a peer that has one", b)
 	}
 }
+
+// A machine with wireguard-tools installed but no tunnel up produces an empty dump. That has
+// to marshal as [] and not as null: the page takes the length of it, and null blanks the
+// whole console rather than saying "no tunnels".
+func TestEmptyDumpIsAnEmptyList(t *testing.T) {
+	for _, dump := range []string{"", "\n", "   \n\n"} {
+		got := parseDump(dump)
+		if got == nil {
+			t.Fatalf("parseDump(%q) = nil, want an empty list", dump)
+		}
+		b, err := json.Marshal(got)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if string(b) != "[]" {
+			t.Errorf("parseDump(%q) marshals to %s, want []", dump, b)
+		}
+	}
+}