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/config/config.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -53,6 +54,9 @@ type Config struct {
 	// Daemon configures the background process (autostart).
 	Daemon Daemon `toml:"daemon"`
 
+	// Notify configures desktop notifications.
+	Notify Notify `toml:"notify"`
+
 	// Lang is the fallback language (de | en) when detection cannot decide,
 	// and the language of talk's own announcements. Default "en"; env
 	// TALK_LANG.
@@ -86,6 +90,13 @@ type Config struct {
 	// QuietMode is the behaviour during quiet hours: "discard" (default,
 	// drop silently) or "toast" (desktop notification instead of audio).
 	QuietMode string `toml:"quiet_mode"`
+
+	// Unknown lists keys in config.toml that talk does not know (dotted,
+	// for example "voice.lang"). Not an error, because an older talk must
+	// still read a newer file, but `talk doctor` reports them: the classic
+	// trap is a top-level key written below a [table] header, which TOML
+	// then files under that table.
+	Unknown []string `toml:"-"`
 }
 
 // Audio configures playback.
@@ -120,6 +131,15 @@ type Daemon struct {
 	Autostart bool `toml:"autostart"`
 }
 
+// Notify holds the desktop notification settings (TOML `[notify]`).
+type Notify struct {
+	// OnSpeak shows a desktop notification for every announcement that is
+	// playing, with a Cancel button where the platform supports it (Linux
+	// with a freedesktop notification service), so a long announcement can
+	// be stopped without a terminal. Default true; env TALK_NOTIFY.
+	OnSpeak bool `toml:"on_speak"`
+}
+
 // Wrap holds the `talk wrap` settings (TOML `[wrap]`).
 type Wrap struct {
 	// Label replaces the command's base name in the announcement, for
@@ -165,6 +185,7 @@ func Defaults() Config {
 		Server:    Server{Port: DefaultServerPort},
 		Queue:     Queue{RateLimit: DefaultRateLimit, AllowHighBurst: true, MaxText: DefaultMaxText, MaxAge: DefaultMaxAge},
 		Daemon:    Daemon{Autostart: true},
+		Notify:    Notify{OnSpeak: true},
 		Lang:      DefaultLang,
 		QuietMode: queue.QuietModeDiscard,
 	}
@@ -202,10 +223,29 @@ func Load(opts Options) (Config, error) {
 		path = filepath.Join(dir, fileName)
 	}
 
-	if _, err := toml.DecodeFile(path, &cfg); err != nil {
+	md, err := toml.DecodeFile(path, &cfg)
+	if err != nil {
 		if !errors.Is(err, os.ErrNotExist) {
 			return cfg, fmt.Errorf("config %s: %w", path, err)
 		}
+	} else {
+		// The decoder lists an unknown table and its keys; keep the keys,
+		// which say more than the table.
+		undecoded := md.Undecoded()
+		for i, key := range undecoded {
+			prefix := key.String() + "."
+			isTable := false
+			for j, other := range undecoded {
+				if i != j && strings.HasPrefix(other.String(), prefix) {
+					isTable = true
+					break
+				}
+			}
+			if !isTable {
+				cfg.Unknown = append(cfg.Unknown, key.String())
+			}
+		}
+		cfg.Unknown = append(cfg.Unknown, strayVoiceKeys(cfg.Voice)...)
 	}
 	return applyEnv(cfg, opts.Getenv)
 }
@@ -260,6 +300,13 @@ func applyEnv(cfg Config, getenv func(string) string) (Config, error) {
 		}
 		cfg.Daemon.Autostart = b
 	}
+	if v := getenv("TALK_NOTIFY"); v != "" {
+		b, err := strconv.ParseBool(v)
+		if err != nil {
+			return cfg, fmt.Errorf("TALK_NOTIFY: invalid value %q (true/false)", v)
+		}
+		cfg.Notify.OnSpeak = b
+	}
 	if v := getenv("TALK_WRAP_LABEL"); v != "" {
 		cfg.Wrap.Label = v
 	}
@@ -326,6 +373,37 @@ func applyEnv(cfg Config, getenv func(string) string) (Config, error) {
 	return cfg, nil
 }
 
+// strayVoiceKeys finds keys under [voice.<provider>] that are not languages
+// (de | en) and voice entries that are not a table of languages at all. The
+// decoder accepts anything there because the mapping is open; a top-level
+// key such as `lang` written below [voice] lands here by accident.
+func strayVoiceKeys(voice map[string]map[string]string) []string {
+	var out []string
+	providers := make([]string, 0, len(voice))
+	for p := range voice {
+		providers = append(providers, p)
+	}
+	sort.Strings(providers)
+	for _, p := range providers {
+		langs := voice[p]
+		if len(langs) == 0 {
+			out = append(out, "voice."+p)
+			continue
+		}
+		keys := make([]string, 0, len(langs))
+		for k := range langs {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		for _, k := range keys {
+			if k != "de" && k != "en" {
+				out = append(out, "voice."+p+"."+k)
+			}
+		}
+	}
+	return out
+}
+
 // splitOrigins splits a comma-separated origin list (the env form of
 // server.allowed_origins, for example "https://a.example, https://b.example").
 // Whitespace around entries is tolerated; empty entries are an error.