feat: FlexRadio TX power per band and mode class

Settings → FlexRadio, beside the per-band antennas and applied the same way:
when the band or the mode changes. 1.5 kW that is fine on SSB has no business
going into an FT8 signal on the same band.

Three classes rather than one row per mode, because what decides the power is
duty cycle and what the amplifier will take of it, not the mode's name — FT8 and
RTTY punish an amplifier the same way, SSB and AM do not. The classification is
pinned by a test: a digital mode sorted as phone would deliver exactly the
1.5 kW this exists to prevent.

An empty box means "leave the power alone", never zero watts, and an unknown
mode changes nothing — setting a transmit power from a name we do not recognise
is not a good guess.

Unlike the antennas, a locked band does not skip it: the lock means "do not let
the rig drag my log entry around", not "let the amplifier keep whatever the last
mode left".

flexpower_test.go

@@ -0,0 +1,25 @@
+package main
+
+import "testing"
+
+// Which class of power a mode falls into.
+//
+// This decides how much power leaves the amplifier, so an error here is not a
+// display bug: a digital mode sorted as phone gets the SSB setting, which is
+// exactly the 1.5 kW into a 100% duty cycle this feature exists to prevent.
+func TestFlexPowerClass(t *testing.T) {
+	for _, c := range []struct{ mode, want string }{
+		{"SSB", "phone"}, {"USB", "phone"}, {"LSB", "phone"}, {"AM", "phone"}, {"FM", "phone"},
+		{"CW", "cw"}, {"cw", "cw"}, {" CW ", "cw"},
+		{"FT8", "digi"}, {"FT4", "digi"}, {"RTTY", "digi"}, {"PSK31", "digi"},
+		{"JS8", "digi"}, {"Q65", "digi"}, {"MSK144", "digi"}, {"DATA", "digi"},
+		{"DIGU", "digi"}, {"DIGL", "digi"}, {"WSPR", "digi"},
+		// Unknown or empty: no class, and the caller changes nothing. Setting a
+		// transmit power from a name we do not recognise is not a good guess.
+		{"", ""}, {"SSTV", ""}, {"BANANA", ""},
+	} {
+		if got := flexPowerClass(c.mode); got != c.want {
+			t.Errorf("flexPowerClass(%q) = %q, want %q", c.mode, got, c.want)
+		}
+	}
+}