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>

README.md

@@ -78,9 +78,9 @@ Keys never go into config.toml.
 - **`talk test`** speaks a fixed test sentence directly in this process (not
   via the daemon; mute and quiet hours do not apply here, an active mute is
   mentioned) and shows which tier spoke and how long synthesis and playback
-  took. `talk test --toast` sends a test desktop notification through the
-  same path `quiet_mode = toast` uses, so you can check notifications
-  without a quiet window or a daemon restart.
+  took. `talk test --toast` shows a test notification with the Cancel button
+  and waits for a click, so you can check the notification setup without
+  speaking anything.
 - **`talk voices`** lists the curated voice catalog (ElevenLabs premade,
   OpenAI, Edge neural voices per locale) as a table; `*` marks the voice
   talk uses without `--voice` (config `voice.<provider>.<lang>` or the
@@ -95,6 +95,26 @@ Keys never go into config.toml.
   very end. Without a terminal (pipe, agent) setup asks nothing and only
   shows the effective configuration.
 
+### Notifications with a Cancel button
+
+Every announcement that plays also shows a desktop notification with its
+text and, on Linux, a **Cancel** button (German: Abbrechen) that stops that
+announcement without a terminal. The notification disappears when the
+announcement ends. talk talks to the freedesktop notification service on the
+session bus directly (any daemon with the "actions" capability: mako, dunst,
+GNOME Shell, KDE, quickshell); without a session bus it falls back to
+notify-send, on macOS to Notification Center and on Windows to a toast, all
+without a button. Turn it off with:
+
+```toml
+[notify]
+on_speak = false   # env TALK_NOTIFY=false
+```
+
+`talk test --toast` shows one such notification, waits a few seconds for a
+click on Cancel and reports what happened; `talk doctor` names the service
+and whether it supports the button.
+
 ### Language
 
 talk detects German and English per announcement (word lists plus umlauts;
@@ -183,11 +203,10 @@ hours are opt-in like mute). During quiet hours audio is dropped, also for
 high (burst is a rate right, not a quiet right):
 
 - `quiet_mode = "discard"` (default): drop silently.
-- `quiet_mode = "toast"`: a desktop notification instead of audio, through
-  the system's own tools (Linux `notify-send`, macOS Notification Center,
-  Windows toast), best effort: if the tool is missing, only the log line
-  remains and `talk doctor` shows it under "Toast". Try it with
-  `talk test --toast`.
+- `quiet_mode = "toast"`: a desktop notification instead of audio (the
+  notification service or the system's own tool, see "Notifications"), best
+  effort: if nothing is available, only the log line remains and
+  `talk doctor` shows it under "Toast".
 - `now:true` **breaks out of quiet hours too** (a deliberate interruption).
 
 **Length and age:** two more laws against what prompts like to do, reading