feat: add tray streaming status and app icon

- tray icon and tooltip now reflect engine state: grey when stopped,
  green + live frame/drop counters in the tooltip while streaming
- generate a proper app icon (assets/, via cmd/teleportfling-icon) and
  embed it so the window and tray carry the icon without runtime files
- add a .desktop entry template for launching from an app menu
- run Fyne UI updates (status label, tray icon/tooltip) on the main
  thread via fyne.Do to satisfy the threading model

internal/gui/icon.go

@@ -0,0 +1,83 @@
+// Package gui provides the Fyne desktop app: a settings window and a system
+// tray that start/stop the flinger engine.
+//
+// The engine itself lives in internal/flinger and is GUI-free, so the tray
+// can control streaming without any UI dependency. This package wires the two
+// together: config persistence, the settings form, the tray menu and the
+// tray status icon.
+package gui
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/png"
+)
+
+// traySize is the icon size in pixels. Tray hosts typically request 22px;
+// we render larger and let the host scale down for crispness.
+const traySize = 64
+
+// newTrayResource renders a rounded square with a status dot into a
+// fyne.Resource that SetSystemTrayIcon accepts.
+func newTrayResource(bg, dot color.NRGBA) *staticResource {
+	img := image.NewRGBA(image.Rect(0, 0, traySize, traySize))
+
+	// Rounded-square background.
+	radius := 14
+	for y := 0; y < traySize; y++ {
+		for x := 0; x < traySize; x++ {
+			if inRoundedRect(x, y, radius) {
+				img.Set(x, y, bg)
+			}
+		}
+	}
+
+	// Center status dot.
+	cx, cy, r := traySize/2, traySize/2, 16
+	for y := cy - r; y <= cy+r; y++ {
+		for x := cx - r; x <= cx+r; x++ {
+			if (x-cx)*(x-cx)+(y-cy)*(y-cy) <= r*r {
+				img.Set(x, y, dot)
+			}
+		}
+	}
+
+	var buf bytes.Buffer
+	_ = png.Encode(&buf, img)
+	return &staticResource{name: "teleportfling-tray", data: buf.Bytes()}
+}
+
+// inRoundedRect reports whether (x,y) falls inside the rounded square.
+func inRoundedRect(x, y, radius int) bool {
+	const m = traySize - 1
+	switch {
+	case x >= radius && x <= m-radius:
+		return true
+	case y >= radius && y <= m-radius:
+		return true
+	}
+	// Corner: within radius of the nearest corner centre.
+	var cx, cy int
+	if x < radius {
+		cx = radius
+	} else {
+		cx = m - radius
+	}
+	if y < radius {
+		cy = radius
+	} else {
+		cy = m - radius
+	}
+	dx, dy := x-cx, y-cy
+	return dx*dx+dy*dy <= radius*radius
+}
+
+// staticResource is a simple in-memory fyne.Resource.
+type staticResource struct {
+	name string
+	data []byte
+}
+
+func (r *staticResource) Name() string    { return r.name }
+func (r *staticResource) Content() []byte { return r.data }