feat: add signal stats panel for selection region

Display Duration, Energy, Peak amplitude, RMS amplitude, and PAPR for
the current time/frequency selection. The stats box:
- Appears next to the selection box (right side, or left if not enough room)
- Centers vertically on the selection box
- Uses a semi-transparent dark background with yellow border
- Updates live while dragging the selection
- Computed in utils.c with ComputeSignalStats()

rspektrum.make

@@ -120,8 +120,10 @@ OBJECTS :=
 
 GENERATED += $(OBJDIR)/spectrogram.o
 GENERATED += $(OBJDIR)/platform_linux.o
+GENERATED += $(OBJDIR)/utils.o
 OBJECTS += $(OBJDIR)/spectrogram.o
 OBJECTS += $(OBJDIR)/platform_linux.o
+OBJECTS += $(OBJDIR)/utils.o
 
 # Rules
 # #############################################
@@ -193,6 +195,10 @@ $(OBJDIR)/platform_linux.o: src/platform_linux.c
 	@echo "$(notdir $<)"
 	$(SILENT) $(CC) $(ALL_CFLAGS) $(FORCE_INCLUDE) -o "$@" -MF "$(@:%.o=%.d)" -c "$<"
 
+$(OBJDIR)/utils.o: src/utils.c
+	@echo "$(notdir $<)"
+	$(SILENT) $(CC) $(ALL_CFLAGS) $(FORCE_INCLUDE) -o "$@" -MF "$(@:%.o=%.d)" -c "$<"
+
 -include $(OBJECTS:%.o=%.d)
 ifneq (,$(PCH))
   -include $(PCH_PLACEHOLDER).d