phraser: report the failure next to the fallback (V-397)

PhraseChat and PhraseQuery returned canned text with a nil error, so a dead
or OOM-killed server was indistinguishable from bad phrasing — "не знаю." is
also a legitimate answer.

Both now return the fallback text AND the error. The daemon keeps using the
text, so the turn still survives; a measuring caller counts a real failure.
An empty response is its own error: the model is up and said nothing.

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

internal/phraser/world.go

@@ -37,6 +37,11 @@ type Remote interface {
 // (docs/evals/2026-08-02-workstation-gemma4-12b.md).
 var ErrNoWorldModel = errors.New("phraser: no world model available")
 
+// errEmptyResponse — the server answered and said nothing. A separate error from
+// a transport failure because it means the model is up and produced no tokens,
+// which is still not an answer and must not score as one.
+var errEmptyResponse = errors.New("phraser: empty response from the model")
+
 // chatTemperature — what the phraser's own transport has always sampled at.
 // Named so the remote path cannot drift from it silently. Whether 0.7 is right
 // at all is Vikunja #402, and answering that here would hide a phrasing change