fix(cache): honest settings card — clean threshold, live stats, no-embedder warning

Full audit of the semantic cache against a live instance (mock embedder,
identical requests): the mechanism works — miss at 3.1s, then hits at 2ms
with entries/hits/saved-$ counting. Two honest gaps fixed:

- the threshold round-tripped through f32 and rendered as
  0.9200000166893005 in both the TOML and the settings input — the config
  field is f64 now (writes exactly what you typed) and the input rounds
  legacy values to 2 decimals;
- the card now shows proof of life (entries / hits / hit-rate / saved $)
  and, when the cache is enabled with NO embedding model available, a
  warning that it is silently doing nothing and how to fix it — previously
  that state looked identical to a working cache.

Also audited in the same pass, no changes needed: Prometheus /metrics
(toggle on/off applies live, 404 when off), and all 7 protocol switches
(each gates its endpoint instantly both ways). 4 strings ×7 languages.

src/cache.rs

@@ -55,7 +55,7 @@ impl SemanticCache {
     pub fn new(cfg: &CacheCfg) -> Arc<Self> {
         Arc::new(Self {
             enabled: cfg.enabled,
-            threshold: cfg.threshold,
+            threshold: cfg.threshold as f32,
             ttl_secs: parse_duration_secs(&cfg.ttl).unwrap_or(3600),
             max_entries: cfg.max_entries.max(1),
             embed_model: cfg.embed_model.clone(),