Stop startup on a list or map setting in neoirc.yaml (closes #268)

Every setting is now read through one helper that uses cast.ToStringE,
the conversion viper.GetString wraps, keeping its error instead of
returning the empty string. TRUSTED_PROXIES uses the same helper. A
config file that does not parse makes New return an error naming the
file instead of panicking.

Model: opus-5-5

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/prometheus/client_golang v1.23.2
 	github.com/rivo/tview v0.42.0
 	github.com/slok/go-http-metrics v0.13.0
+	github.com/spf13/cast v1.10.0
 	github.com/spf13/viper v1.21.0
 	go.uber.org/fx v1.24.0
 	go4.org/netipx v0.0.0-20260823151212-3075585bcbeb
@@ -41,7 +42,6 @@ require (
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
 	github.com/spf13/afero v1.15.0 // indirect
-	github.com/spf13/cast v1.10.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
 	go.uber.org/dig v1.19.0 // indirect