Fix T1000-E negative temperature display bug

The ntc_temp2 lookup table was declared as 'char', which can be
unsigned on some platforms, causing negative temperature values
(-30°C to -1°C) to be incorrectly interpreted.

Changed to int8_t to ensure proper signed integer handling of
negative temperatures.

Fixes #1389

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

variants/t1000-e/t1000e_sensors.cpp

@@ -21,7 +21,7 @@ static unsigned int ntc_res2[136] = {
   1081,   1053,   1026,   999,   974,   949,   925,   902,   880,   858,
 };
 
-static char ntc_temp2[136] = {
+static int8_t ntc_temp2[136] = {
   -30, -29, -28, -27, -26, -25, -24, -23, -22, -21, -20, -19, -18, -17, -16, -15, -14, -13, -12, -11,
   -10, -9,  -8,  -7,  -6,  -5,  -4,  -3,  -2,  -1,  0,   1,   2,   3,   4,   5,   6,   7,   8,   9,
   10,  11,  12,  13,  14,  15,  16,  17,  18,  19,  20,  21,  22,  23,  24,  25,  26,  27,  28,  29,