[test](trx-frontend-http): serve a realistic decoder registry to the smoke test

The fixture answered /decoders with an empty list, which hid most of the
application from the only test that runs it in a browser.  The decoder
sub-tabs, their panels, the decode toggles and the bookmark decoder
checkboxes are all built from that registry, so the run exercised three
of thirteen sub-tabs and none of the decoder UI.  Finding this needed
route interception, because nothing in the suite could see it.

Serve eleven decoders covering the modes the real registry spans.  The
run now builds 13 sub-tabs and 11 bookmark decoder checkboxes — the same
checkboxes whose construction a recent fix changed without any test
reaching them — and still reports no runtime errors.

It also makes an existing fault observable: at 1100px the decoder
sub-tab bar hides 195px of itself with no scrollbar or fade, the same
silent truncation the top strip had.  No assertion for it here, since
that would fail until the truncation is fixed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GdyUjuXejCEfiub675z6cz
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/frontend/tests/browser-smoke.mjs

@@ -30,9 +30,28 @@ const rigItems = ["rig-a", "rig-b"].map((remote) => ({
 const rigsResponse = { rigs: rigItems, active_remote: "rig-a" };
 const selectedRigs = [];
 
+// A realistic decoder registry.  Serving an empty one hid most of the
+// application from this test: the decoder sub-tabs, their panels, the decode
+// toggles and the bookmark decoder checkboxes are all built from it, so with
+// no decoders only three of thirteen sub-tabs existed and none of the decoder
+// UI was ever constructed.
+const decoderRegistry = [
+  { id: "ft8", label: "FT8", activation: "toggle", active_modes: ["USB"] },
+  { id: "ft4", label: "FT4", activation: "toggle", active_modes: ["USB"] },
+  { id: "ft2", label: "FT2", activation: "toggle", active_modes: ["USB"] },
+  { id: "wspr", label: "WSPR", activation: "toggle", active_modes: ["USB"] },
+  { id: "cw", label: "CW", activation: "toggle", active_modes: ["CW", "CWR"] },
+  { id: "sat", label: "SAT", activation: "toggle", active_modes: ["FM"] },
+  { id: "wefax", label: "WEFAX", activation: "toggle", active_modes: ["USB"] },
+  { id: "ais", label: "AIS", activation: "toggle", active_modes: ["FM"] },
+  { id: "aprs", label: "APRS", activation: "toggle", active_modes: ["FM"] },
+  { id: "hf-aprs", label: "HF APRS", activation: "toggle", active_modes: ["USB"] },
+  { id: "vdes", label: "VDES", activation: "toggle", active_modes: ["FM"] },
+].map((decoder) => ({ ...decoder, background_decode: false, bookmark_selectable: true }));
+
 const jsonRoutes = new Map([
   ["/auth/session", { authenticated: true, role: "control", auth_disabled: true }],
-  ["/decoders", []],
+  ["/decoders", decoderRegistry],
   ["/rigs", rigsResponse],
   ["/status", {
     info: {