Phase 2b: server/client timeouts, atomic state writes, log mutex, panic and shutdown fixes

- http.Server with ReadHeaderTimeout/IdleTimeout (no read/write timeouts: /stream/ is long-lived)
- shared outbound clients: providerHTTPClient (5m), apiHTTPClient (30s), imgcache client (30s)
- imgcache: download outside the lock, per-item helper, cache URL uses the file name not the fs path
- writeFileAtomic (temp + fsync + rename) for settings/xepg/pms/urls/authentication JSON
- one package-level logMu for WebScreenLog and notifications; ring buffer keeps the newest lines
- notifications evict the oldest instead of random map entries
- xepg XMLTV file removal rebuilt after the loop; data.go range-mutation removed
- API handler returns after error body; WS request/response fresh per command
- checked type assertions in data/backup/provider/screen
- SIGINT/SIGTERM handled in main via src.Shutdown(); fatal paths exit 1

src/toolchain_test.go

@@ -0,0 +1,86 @@
+package src
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+func TestWriteFileAtomic(t *testing.T) {
+	dir := t.TempDir()
+	file := filepath.Join(dir, "settings.json")
+
+	if err := writeFileAtomic(file, []byte("first"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := writeFileAtomic(file, []byte("second"), 0600); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := os.ReadFile(file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "second" {
+		t.Errorf("content = %q, want %q", got, "second")
+	}
+
+	fi, err := os.Stat(file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Mode().Perm() != 0600 {
+		t.Errorf("mode = %o, want 0600", fi.Mode().Perm())
+	}
+
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, e := range entries {
+		if e.Name() != "settings.json" {
+			t.Errorf("unexpected leftover file %q", e.Name())
+		}
+	}
+}
+
+func TestWriteFileAtomicMissingDir(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "missing", "settings.json")
+	if err := writeFileAtomic(file, []byte("x"), 0644); err == nil {
+		t.Fatal("expected an error for a missing directory")
+	}
+}
+
+func TestRemoveStrings(t *testing.T) {
+	list := []string{"a.xml", "b.xml", "c.xml", "d.xml"}
+	got := removeStrings(list, map[string]bool{"b.xml": true, "d.xml": true})
+	want := []string{"a.xml", "c.xml"}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("removeStrings = %v, want %v", got, want)
+	}
+
+	// Nothing to remove: same content, original untouched.
+	got = removeStrings(list, nil)
+	if !reflect.DeepEqual(got, list) {
+		t.Errorf("removeStrings(nil) = %v, want %v", got, list)
+	}
+}
+
+func TestToFloat64(t *testing.T) {
+	cases := []struct {
+		in   any
+		want float64
+	}{
+		{float64(3), 3},
+		{int(2), 2},
+		{int64(1), 1},
+		{"x", 0},
+		{nil, 0},
+	}
+	for _, c := range cases {
+		if got := toFloat64(c.in); got != c.want {
+			t.Errorf("toFloat64(%v) = %v, want %v", c.in, got, c.want)
+		}
+	}
+}