feat(icon): app icon with embedded PNG, Windows .ico and resource objects

One SVG source (assets/icon/talk.svg: speech bubble with level bars).
assets/icon/gen.go renders it with inkscape into the embedded PNG
(internal/icon, installed next to the config for notifications), the
multi-size talk.ico and the COFF .rsrc objects for windows/amd64 and
arm64 that give talk.exe its icon. The generator writes the resource
objects itself, so no resource compiler is needed; everything it
produces is committed and a plain go build picks it up.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/icon/icon_test.go

@@ -0,0 +1,49 @@
+package icon
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestInstall: the icon lands in the directory, is left alone when current,
+// and is repaired when the file on disk differs.
+func TestInstall(t *testing.T) {
+	if len(PNG) < 8 || !bytes.HasPrefix(PNG, []byte("\x89PNG")) {
+		t.Fatalf("embedded icon is not a PNG (%d bytes)", len(PNG))
+	}
+	dir := filepath.Join(t.TempDir(), "talk")
+	path, err := Install(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if path != filepath.Join(dir, FileName) {
+		t.Errorf("path = %q", path)
+	}
+	got, err := os.ReadFile(path)
+	if err != nil || !bytes.Equal(got, PNG) {
+		t.Fatalf("installed file differs (err %v)", err)
+	}
+	first, _ := os.Stat(path)
+	if _, err := Install(dir); err != nil {
+		t.Fatal(err)
+	}
+	second, _ := os.Stat(path)
+	if !second.ModTime().Equal(first.ModTime()) {
+		t.Error("unchanged icon was rewritten")
+	}
+	if err := os.WriteFile(path, []byte("stale"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := Install(dir); err != nil {
+		t.Fatal(err)
+	}
+	got, _ = os.ReadFile(path)
+	if !bytes.Equal(got, PNG) {
+		t.Error("stale icon was not replaced")
+	}
+	if _, err := Install(""); err == nil {
+		t.Error("Install(\"\") succeeded")
+	}
+}