feat(cw): show tone-shift markers on the waterfall spectrogram

When the tone-shift feature moves a tone into the model's 400-1200 Hz window,
the waterfall now shows two visual markers so the operator can see what is
happening: a green dashed line at the target (800 Hz) and an orange frequency
label at the top-left showing the original pitch.

The waterfall draws the RAW audio, not the shifted audio, so a 1500 Hz tone was
always invisible regardless of the shift setting. The markers close the gap:
the operator can now see that a tone was detected and where it was moved, even
when the original pitch is outside the visible band.

activeShiftHz is now a StateFlow exposed through ICwDecoder so the UI can
observe it without polling.

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/cw/ICwDecoder.kt

@@ -46,6 +46,9 @@ interface ICwDecoder {
     /** Detected tone frequency in Hz, or null before a tone is found. */
     val estimatedPitch: StateFlow<Float?>
 
+    /** Current shift applied to bring the tone into the model's window, 0f when idle. */
+    val activeShiftHz: StateFlow<Float>
+
     /** Relative signal strength in 0..1 for level meters. */
     val signalStrength: StateFlow<Float>