Fix WebUI rolling-window and buffer-wrap display bugs

Replace fragile zoomGuard boolean with userInteracting flag so that
programmatic scale updates (rolling window, resize, fit) never lock
p.xRange.  Only genuine mouse drag or scroll-wheel events on the uPlot
canvas set userInteracting=true and allow onZoom to freeze the view.

Also move stale-xRange detection out of the needsRedraw gate so that a
plot whose circular buffer has scrolled past a frozen zoom range
automatically returns to rolling-window mode every frame, fixing the
second bug where data disappeared as the buffer wrapped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Client/WebUI/protocol.go

@@ -19,15 +19,21 @@ const (
 	PktConnect    uint8 = 3
 	PktDisconnect uint8 = 4
 
-	HeaderSize     = 17
-	SigDescSize    = 136
-	NoTimeSignal   = uint32(0xFFFFFFFF)
+	HeaderSize   = 17
+	SigDescSize  = 136
+	NoTimeSignal = uint32(0xFFFFFFFF)
 
 	QuantNone   uint8 = 0
 	QuantUint8  uint8 = 1
 	QuantInt8   uint8 = 2
 	QuantUint16 uint8 = 3
 	QuantInt16  uint8 = 4
+
+	// TimeMode values – must match UDPStreamerTimeMode enum in UDPStreamer.h
+	TimeModePacket      uint8 = 0 // use wall-clock packet arrival time
+	TimeModeFullArray   uint8 = 1 // TimeSignal has same N elements; not expanded here
+	TimeModeFirstSample uint8 = 2 // TimeSignal scalar = time of element [0]
+	TimeModeLastSample  uint8 = 3 // TimeSignal scalar = time of element [N-1]
 )
 
 // ─── Packet header (17 bytes, little-endian, packed) ─────────────────────────