[feat](trx-frontend-http): use monospace font for signal strength units

Wrap unit labels (dBm, dBf, dBFS, S, dB) in a .sig-unit span styled
with the system monospace stack, keeping numeric values in DSEG14.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -662,6 +662,11 @@ button:disabled { opacity: 0.6; cursor: not-allowed; }
   cursor: pointer;
   user-select: none;
 }
+.sig-unit {
+  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
+  font-size: 0.85em;
+  font-weight: 400;
+}
 small { color: var(--text-muted); }
 .header {
   margin-bottom: 0;