feat(photos): добавить пресеты кривой и починить переключение фото

- PCHIP для предпросмотра и серверного рендера
- пресеты всех каналов в профиле, импорт и экспорт
- мерж настроек auth без потери разделов
- сохранение вкладки и правок текущего фото при переключении

backend/internal/photos/edit_test.go

@@ -91,6 +91,70 @@ func TestCurveLUTSteepens(t *testing.T) {
 	}
 }
 
+// The client builds the same 256-entry LUT (frontend/src/lib/edit.ts). The
+// hard-coded samples keep the Go and TypeScript implementations in sync.
+func TestCurveLUTMatchesClient(t *testing.T) {
+	samples := []int{0, 32, 64, 128, 192, 224, 255}
+	cases := []struct {
+		name   string
+		points []CurvePoint
+		want   []int
+	}{
+		{"linear", []CurvePoint{{0, 0}, {1, 1}}, []int{0, 32, 64, 128, 192, 224, 255}},
+		{
+			"medium",
+			[]CurvePoint{{0, 0}, {0.25, 0.2}, {0.5, 0.5}, {0.75, 0.8}, {1, 1}},
+			[]int{0, 23, 51, 128, 205, 233, 255},
+		},
+		{
+			"strong",
+			[]CurvePoint{{0, 0}, {0.2, 0.12}, {0.5, 0.5}, {0.8, 0.88}, {1, 1}},
+			[]int{0, 17, 43, 128, 213, 239, 255},
+		},
+		{
+			"inner",
+			[]CurvePoint{{0.1, 0.05}, {0.4, 0.3}, {0.9, 0.95}},
+			[]int{13, 17, 42, 104, 186, 234, 242},
+		},
+	}
+	for _, tc := range cases {
+		lut := curveLUT(tc.points)
+		for i, index := range samples {
+			if int(lut[index]) != tc.want[i] {
+				t.Fatalf("%s lut[%d] = %d, want %d", tc.name, index, lut[index], tc.want[i])
+			}
+		}
+	}
+}
+
+func TestCurveLUTIsMonotone(t *testing.T) {
+	for _, points := range [][]CurvePoint{
+		{{0, 0}, {0.25, 0.2}, {0.5, 0.5}, {0.75, 0.8}, {1, 1}},
+		{{0, 0}, {0.2, 0.12}, {0.5, 0.5}, {0.8, 0.88}, {1, 1}},
+	} {
+		lut := curveLUT(points)
+		for i := 1; i < len(lut); i++ {
+			if lut[i] < lut[i-1] {
+				t.Fatalf("lut[%d] = %d < lut[%d] = %d", i, lut[i], i-1, lut[i-1])
+			}
+		}
+	}
+}
+
+func TestCurveLUTFlatOutsideEndpoints(t *testing.T) {
+	lut := curveLUT([]CurvePoint{{0.1, 0.05}, {0.4, 0.3}, {0.9, 0.95}})
+	for i := 0; i <= int(math.Floor(0.1*255)); i++ {
+		if lut[i] != 13 {
+			t.Fatalf("lut[%d] = %d, want flat 13", i, lut[i])
+		}
+	}
+	for i := 230; i < len(lut); i++ {
+		if lut[i] != 242 {
+			t.Fatalf("lut[%d] = %d, want flat 242", i, lut[i])
+		}
+	}
+}
+
 func TestApplyLightBrightensAndKeepsSource(t *testing.T) {
 	src := image.NewNRGBA(image.Rect(0, 0, 2, 2))
 	for i := range src.Pix {