Make /reminders the complete cancellation surface (V-719)

GET merges every pending reminder, ordered by next fire, with the latest
50 rows and no duplicates, so old pending work cannot fall off a history
window. Recurring rows show their next fire and cron expression.

A pending row carries an inline cancel POST. Success answers 303 so a
refresh cannot repeat the mutation. A missing id is 404, a terminal or
in-flight row is 409, a malformed id or action is 400, and a transport
failure keeps the sanitized 502 problem response.

The page calls the same CoreAPI methods the voice path uses rather than
opening a second route into the store.

--no-verify: master is the working branch this session by the owner's call.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

cmd/mavweb/main.go

@@ -143,7 +143,9 @@ func main() {
 		log.Printf("mavweb: ambient notification ingest enabled at POST /api/ambient")
 	}
 
-	// The read surfaces. Every one of them 503s without -core.
+	// The data surfaces. Every one of them 503s without -core. /reminders also
+	// accepts an ID-bound cancellation POST. It is deliberately not step-up
+	// gated: like dismissing a proposed routine, it can only make Maven quieter.
 	mux.HandleFunc("/dash", corePage(handleDash))
 	mux.HandleFunc("/history", corePage(handleHistory))
 	mux.HandleFunc("/trace", corePage(handleTrace))