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

cmd/teleportfling-icon/main.go

@@ -0,0 +1,115 @@
+// Command teleportfling-icon regenerates the application icon assets in
+// assets/ from scratch. It draws a rounded green square with two lighter
+// "fling" dots suggesting motion.
+//
+// Usage:
+//
+//	teleportfling-icon
+//
+// The generated PNGs are committed to the repo and embedded into the binary
+// via the assets package; this tool exists so the icon can be tweaked and
+// regenerated without a design tool.
+package main
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/png"
+	"os"
+)
+
+// sizes are the icon sizes to emit. Hosts scale from these; 512 is used for
+// the app icon, 256 and 64 for smaller contexts.
+var sizes = []int{512, 256, 64}
+
+// outputDir is where the PNGs are written (the repo assets directory).
+const outputDir = "assets"
+
+// inRoundedRect reports whether (x,y) lies inside a square with rounded
+// corners of the given radius.
+func inRoundedRect(x, y, radius, size int) bool {
+	m := size - 1
+	switch {
+	case x >= radius && x <= m-radius:
+		return true
+	case y >= radius && y <= m-radius:
+		return true
+	}
+	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
+}
+
+// render draws the icon at the given size into a PNG.
+func render(size int, bg, dot color.NRGBA) []byte {
+	img := image.NewNRGBA(image.Rect(0, 0, size, size))
+	radius := size / 5
+	for y := 0; y < size; y++ {
+		for x := 0; x < size; x++ {
+			if inRoundedRect(x, y, radius, size) {
+				img.SetNRGBA(x, y, bg)
+			}
+		}
+	}
+	// Two dots suggesting a flung stream, offset diagonally.
+	for i, off := range []int{3, 9} {
+		cx, cy, r := size/2, size/2+off*size/32, size/8-i
+		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.SetNRGBA(x, y, dot)
+				}
+			}
+		}
+	}
+	var buf bytes.Buffer
+	_ = png.Encode(&buf, img)
+	return buf.Bytes()
+}
+
+func main() {
+	bg := color.NRGBA{R: 46, G: 125, B: 50, A: 255} // teleport green
+	dot := color.NRGBA{R: 150, G: 220, B: 140, A: 255}
+
+	//nolint:gosec // writing generated PNG assets into the repo, not secrets
+	if err := os.MkdirAll(outputDir, 0o755); err != nil {
+		panic(err)
+	}
+
+	for _, size := range sizes {
+		name := "teleportfling"
+		if size != 512 {
+			name += "-" + itoa(size)
+		}
+		path := outputDir + "/" + name + ".png"
+		//nolint:gosec // generated PNG assets, world-readable for installers
+		if err := os.WriteFile(path, render(size, bg, dot), 0o644); err != nil {
+			panic(err)
+		}
+	}
+}
+
+func itoa(v int) string {
+	if v == 0 {
+		return "0"
+	}
+	var buf [10]byte
+	i := len(buf)
+	for v > 0 {
+		i--
+		buf[i] = byte('0' + v%10)
+		v /= 10
+	}
+	return string(buf[i:])
+}