feat(notify): Windows toasts under talk's own identity, icon on Linux

The daemon (and talk test --toast) now register the AppUserModelID
m3ta.talk under HKCU with display name and the installed icon, so
toasts appear as "talk" with the icon instead of Windows PowerShell;
the PowerShell identity stays as fallback until the registration has
run once. The redundant "talk" title line is dropped under the own
identity. notify.Setup bundles icon install, identity and the talk:
protocol handler for both callers. Linux notifications carry the
installed icon as app_icon. doctor names the active identity.

Verified on the Windows 11 VM: registry key, icon file, toast with
working Cancel button, exe icon; Linux via quickshell.

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

internal/notify/notify_test.go

@@ -66,3 +66,67 @@ func TestAppleString(t *testing.T) {
 		t.Errorf("appleString = %s", got)
 	}
 }
+
+// TestToastIdentity: under talk's own AppUserModelID the toast header names
+// talk already, so the scripts carry only the body; under the PowerShell
+// fallback identity a first line names talk.
+func TestToastIdentity(t *testing.T) {
+	a := Announcement{Text: "build done", CancelLabel: "Cancel", CancelURI: "talk:cancel?id=1", Tag: "1"}
+	own := cancelToastScript(a, "talk-1", AppID)
+	if !strings.Contains(own, "CreateToastNotifier('"+AppID+"')") || strings.Contains(own, "<text>"+Title+"</text>") {
+		t.Errorf("own identity script wrong:\n%s", own)
+	}
+	fallback := cancelToastScript(a, "talk-1", powerShellAppID)
+	if !strings.Contains(fallback, "CreateToastNotifier('"+powerShellAppID+"')") || !strings.Contains(fallback, "<text>"+Title+"</text>") {
+		t.Errorf("fallback script wrong:\n%s", fallback)
+	}
+	plain := toastScript("quiet", AppID)
+	if strings.Contains(plain, "<text>"+Title+"</text>") || !strings.Contains(plain, "<text>$msg</text>") {
+		t.Errorf("plain own-identity script wrong:\n%s", plain)
+	}
+	if !strings.Contains(toastScript("quiet", powerShellAppID), "<text>"+Title+"</text>") {
+		t.Error("plain fallback script lacks the talk line")
+	}
+}
+
+// TestUseIconOutsideWindows: UseIcon remembers the file and the freedesktop
+// notification carries it as app_icon; without one a theme icon is used.
+func TestUseIconOutsideWindows(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("registry path on Windows")
+	}
+	setIconPath("")
+	if appIcon() != "audio-speakers" {
+		t.Errorf("appIcon without file = %q", appIcon())
+	}
+	if err := UseIcon("/x/talk.png"); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { setIconPath("") })
+	if appIcon() != "/x/talk.png" {
+		t.Errorf("appIcon = %q", appIcon())
+	}
+	if ownIdentity() {
+		t.Error("ownIdentity true outside Windows")
+	}
+}
+
+// TestSetup: the icon lands in dir and is used; with no exe the protocol
+// step is skipped (outside Windows it is a no-op anyway).
+func TestSetup(t *testing.T) {
+	dir := t.TempDir()
+	if err := Setup(dir, ""); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { setIconPath("") })
+	want := filepath.Join(dir, "talk.png")
+	if _, err := os.Stat(want); err != nil {
+		t.Fatalf("icon not installed: %v", err)
+	}
+	if runtime.GOOS != "windows" && appIcon() != want {
+		t.Errorf("appIcon = %q, want %q", appIcon(), want)
+	}
+	if err := Setup("", ""); err == nil {
+		t.Error("Setup without dir succeeded")
+	}
+}