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.

web/js/i18n/de.js

@@ -635,4 +635,8 @@ export default {
   "tier.medium": "mittel",
   "tier.high": "schwer",
   "bm.scoreHint": "Der kontinuierliche Komplexitätsscore des Routers (0..1) — die Grenzregler auf der Routing-Seite teilen ihn in Stufen.",
+  "settings.cache.noEmbed": "Der Cache ist aktiviert, hat aber KEIN Embedding-Modell — er tut nichts. Fügen Sie dem Pool ein Modell mit kind = \"embedding\" hinzu (z. B. OpenAI text-embedding) oder wählen Sie unten eines.",
+  "settings.cache.entries": "Einträge",
+  "settings.cache.hits": "Treffer",
+  "settings.cache.saved": "gespart",
 };