phraser: say "даже не знаю, что сказать" when there is nothing to say (V-397)

Review of #108: "поговорили." reads as a summary of a conversation that did
not happen. One exported constant now, so the Stub, the LLMPhraser fallback
and the daemon all say the same thing.

internal/voice/replier.go keeps its own copy — that is the separate replier
seam, not this one.

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

internal/phraser/failure_test.go

@@ -9,7 +9,7 @@ import (
 )
 
 // A dead server must be distinguishable from bad phrasing. Both PhraseChat and
-// PhraseQuery keep the turn alive with canned text — "поговорили.", "не знаю.",
+// PhraseQuery keep the turn alive with canned text — ChatFallback, "не знаю.",
 // "вот что я нашла: …" — and every one of those is also a legitimate reply, so
 // the text alone cannot say which happened. The error is the only signal, and
 // before Vikunja #397 it was dropped: the talk scorer reported a full run with
@@ -28,7 +28,7 @@ func TestPhrasingReportsTheFailureWithTheFallback(t *testing.T) {
 	}{
 		{"chat", func() (string, error) {
 			return p.PhraseChat(context.Background(), "как дела", nil)
-		}, "поговорили."},
+		}, ChatFallback},
 		{"knowledge", func() (string, error) {
 			return p.PhraseQuery(context.Background(), "кто написал войну и мир", nil)
 		}, "не знаю."},