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/identity_other.go

@@ -0,0 +1,13 @@
+//go:build !windows
+
+package notify
+
+// UseIcon remembers the icon file for notifications (Linux: app_icon of the
+// freedesktop notification). macOS has no way to pass one to osascript.
+func UseIcon(iconPath string) error {
+	setIconPath(iconPath)
+	return nil
+}
+
+// ownIdentity is a Windows concept (toast AppUserModelID).
+func ownIdentity() bool { return false }