feat(notify): desktop notification per announcement with a Cancel button

Every playing announcement now shows a desktop notification with its text
and, on Linux, a Cancel button (Abbrechen for German) that stops exactly
that announcement; the notification is closed when the announcement ends.
This brings back what the old talk.sh had with notify-send --action, as a
default: a long announcement can be cut short without a terminal.

Implementation: the freedesktop Notifications service on the session bus
via godbus (pure Go): one connection per process, Notify with the action
list, ActionInvoked dispatched to the announcement, CloseNotification at
the end, expire_timeout 0 so the button stays for the whole announcement.
Without a session bus the notify-send fallback shows a plain notification;
macOS and Windows show one without a button for now. queue.CancelItem(id)
cancels only the playing item. Config [notify] on_speak (default true, env
TALK_NOTIFY). `talk test --toast` now shows the real thing and waits for a
click; `talk doctor` names the service and whether it supports the button.

Config hygiene found on the way: a key written below a [table] header
belongs to that table in TOML (lang under [voice] was silently ignored).
config.Load now collects unknown keys (undecoded ones and non-language
keys under [voice.<provider>]) and `talk doctor` warns with the hint.

Tests: process test on a private dbus-daemon with a fake
org.freedesktop.Notifications service (Notify payload, cancel click aborts
the synthesis, CloseNotification), queue.CancelItem, config keys. All test
binaries point DBUS_SESSION_BUS_ADDRESS at a dead socket so no test ever
pops a notification on the developer's desktop. flake vendorHash updated
for the new dependency.

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

internal/queue/queue.go

@@ -259,6 +259,19 @@ func (q *Queue) Cancel() int {
 	return n
 }
 
+// CancelItem aborts the announcement with the given id if it is the one
+// playing right now (the Cancel button of its notification). Waiting items
+// and the rest of the queue are untouched. Reports whether it hit.
+func (q *Queue) CancelItem(id string) bool {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	if q.current == nil || q.current.ID != id || q.cancelCur == nil {
+		return false
+	}
+	q.cancelCur()
+	return true
+}
+
 // Close shuts the queue down: waiting items are dropped, the playing
 // announcement is aborted, the worker loop ends. Close waits until the loop
 // has ended (idempotent).