[fix](trx-rs): show signal strength with decimal precision

Change RigRxStatus.sig from i32 to f64 and add get_signal_strength_db
to RigCat trait so SDR backends can bypass the coarse 0..15 quantisation.
Compensate for decimation processing gain so the meter matches the
spectrum peak. Display with one decimal place in all units.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -582,16 +582,16 @@ let sigStrengthUnitIdx = loadSetting("sigStrengthUnit", 0);
 function formatSigStrength(dbm) {
   if (!Number.isFinite(dbm)) return "--";
   const unit = SIG_STRENGTH_UNITS[sigStrengthUnitIdx] || "dBFS";
-  if (unit === "dBm") return `${dbm} dBm`;
+  if (unit === "dBm") return `${dbm.toFixed(1)} dBm`;
   if (unit === "dBf") {
     // dBf = dBm + 107 (referenced to 1 femtowatt across 50 Ω)
     const dbf = dbm + 107;
-    return `${dbf.toFixed(0)} dBf`;
+    return `${dbf.toFixed(1)} dBf`;
   }
   // dBFS: map receiver range to a full-scale reference
   // Typical receiver: -140 dBm (noise floor) to 0 dBm (full scale)
   const dbfs = Math.max(-140, Math.min(0, dbm));
-  return `${dbfs} dBFS`;
+  return `${dbfs.toFixed(1)} dBFS`;
 }
 
 function refreshSigStrengthDisplay() {
@@ -9199,7 +9199,7 @@ function startSpectrumStreaming() {
       vchanRdsById = next;
       vchanSignalDbById = nextSig;
       if (typeof vchanActiveId !== "undefined" && vchanActiveId && nextSig.has(vchanActiveId)) {
-        sigLastDbm = Math.round(nextSig.get(vchanActiveId));
+        sigLastDbm = nextSig.get(vchanActiveId);
         refreshSigStrengthDisplay();
       }
       updateRdsPsOverlay(primaryRds);