was: make notification cancellation atomic

Notification callbacks set notify_data.cancel while notify_task reads
it without holding notify_mutex. The plain bool access is therefore a
data race even though updates of notify_active itself are serialized.

Make only the cancel flag C11 atomic. Existing assignments and reads
become sequentially consistent atomic operations, retaining the current
control flow and cancellation behavior while removing undefined
behavior.

Reported-by: GPT-5.6-sol

Author: stintel

main/was.c

@@ -36,7 +36,7 @@ struct notify_data {
     char *audio_url;
     bool backlight;
     bool backlight_max;
-    bool cancel;
+    _Atomic bool cancel;
     char *text;
     int repeat;
     int strobe_period_ms;