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/views/settings.js

@@ -194,8 +194,30 @@ export async function renderSettings() {
   const modelsData = await api.listModels();
   const embedModels = (modelsData.models || []).filter((m) => m.kind === "embedding").map((m) => m.id);
   const cacheEnabled = check(t("settings.cache.enabled"), s.cache?.enabled, t("settings.cache.enabledHelp"));
-  const cacheThresh = h("input", { type: "number", step: "0.01", value: s.cache?.threshold ?? 0.92 });
+  const cacheThresh = h("input", { type: "number", step: "0.01", min: "0.5", max: "0.999", value: Math.round(((s.cache?.threshold ?? 0.92) + Number.EPSILON) * 100) / 100 });
   const cacheTtlIn = h("input", { value: s.cache?.ttl || "1h" });
+  // Live proof-of-life for the cache card: entries/hits/saved — and an
+  // honest warning when the cache is on but has no embedding model to work
+  // with (it silently does nothing in that state).
+  let cacheStats = null;
+  try {
+    cacheStats = (await api.stats("24h", "model")).cache || null;
+  } catch { /* stats off */ }
+  function cacheStatus() {
+    const wrap = h("div", { style: "margin-bottom:10px" });
+    if (s.cache?.enabled && embedModels.length === 0 && !s.cache?.embed_model) {
+      mount(wrap, h("div", { class: "callout warn" }, "⚠ " + t("settings.cache.noEmbed")));
+      return wrap;
+    }
+    if (cacheStats && s.cache?.enabled) {
+      mount(wrap, h("div", { class: "flex wrap", style: "gap:8px" },
+        h("span", { class: "badge" }, t("settings.cache.entries") + ": " + cacheStats.entries),
+        h("span", { class: "badge ok" }, t("settings.cache.hits") + ": " + cacheStats.hits),
+        h("span", { class: "badge" }, "hit-rate " + Math.round((cacheStats.hit_rate || 0) * 100) + "%"),
+        h("span", { class: "badge" }, t("settings.cache.saved") + " $" + (cacheStats.saved_usd || 0).toFixed(4))));
+    }
+    return wrap;
+  }
   const cacheEmbedSel = h(
     "select",
     {},
@@ -610,6 +632,7 @@ export async function renderSettings() {
         "div",
         { class: "card" },
         h("div", { class: "card-head" }, h("h3", {}, t("settings.cache"))),
+        cacheStatus(),
         cacheEnabled.node,
         h("div", { class: "row" }, field(t("settings.cache.threshold"), cacheThresh), field(t("settings.cache.ttl"), cacheTtlIn)),
         field(t("settings.cache.embed"), cacheEmbedSel),