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/en.js

@@ -628,4 +628,8 @@ export default {
   "tier.medium": "medium",
   "tier.high": "high",
   "bm.scoreHint": "The router's continuous complexity score (0..1) — the boundary sliders on the Routing page cut it into tiers.",
+  "settings.cache.noEmbed": "The cache is enabled but has NO embedding model to work with — it is doing nothing. Add a model with kind = \"embedding\" to the pool (e.g. an OpenAI text-embedding model) or pick one below.",
+  "settings.cache.entries": "entries",
+  "settings.cache.hits": "hits",
+  "settings.cache.saved": "saved",
 };