[feat](trx-rs): receive SSTV pictures end to end

Wires the SSTV decoder into the stack, from the audio the server already
has to a panel in the browser that shows the picture arriving.

Server: a decoder task alongside the WEFAX one, running whenever the
decoder is enabled and the rig is in a mode SSTV is sent in.  A finished
picture is written to the cache as a PNG and sent on as a message; the
rows are sent as they decode, so a client can watch two minutes of
Martin M1 fill in rather than waiting for it.  Pictures join the decode
history, are replayed to a client that connects later, and survive a
restart.

Protocol: SetSstvDecodeEnabled and ResetSstvDecoder, a sstv_decode
_enabled flag in the rig state, two audio message types, and Sstv and
SstvProgress on DecodedMessage.  The history stores the message without
its base64 payload -- the picture is already on disk, and a megabyte per
entry is not what a history is for.

Client: pictures land in their own history, and the PNG the server sent
is written to the local cache so /sstv-images/ can serve it back.  That
endpoint and the WEFAX one now share their filename checks rather than
each carrying a copy: no separators, no parent references, .png only.

Web UI: an SSTV sub-tab beside WEFAX, with a live canvas the rows paint
into at the line number they carry, a card for the last picture, and a
filterable history with links to the files.  Rows below the one arriving
are grey rather than black -- not yet received is a different thing from
received as black.  A picture is not a spot, so neither pictures nor
their progress updates reach the decode statistics; that exclusion list
had grown by hand for LRPT and WEFAX and is now one named set.

The decoder crate gains what the server needed to hand a picture on:
to_png, to_png_base64 and save_png, with file names stamped in UTC so
they sort.

Panel behaviour is tested with the plugin runtime: rows painting at
their own line numbers rather than in arrival order, a completed picture
linked by file name alone with no server path in the page, a cut-off
picture reported as partial, clearing, and the toggle following the rig
state.

Signed-off-by: Stan Grams <sjg@haxx.space>

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

@@ -932,7 +932,7 @@ function elementById(id) {
       ["Overview", ["overview"]],
       ["Marine & packet", ["ais", "vdes", "aprs", "hf-aprs"]],
       ["Weak signal", ["cw", "ft8", "ft4", "ft2", "wspr"]],
-      ["Broadcast & images", ["rds", "sat", "wefax"]]
+      ["Broadcast & images", ["rds", "sat", "wefax", "sstv"]]
     ];
     groups.forEach(([label, ids]) => {
       const group = document.createElement("optgroup");
@@ -1748,6 +1748,7 @@ var pluginGroups = {
     "/background-decode.js",
     "/sat.js",
     "/wefax.js",
+    "/sstv.js",
     "/ais.js",
     "/vdes.js",
     "/aprs.js",
@@ -2240,7 +2241,7 @@ function currentDecodeHistoryRetentionMs() {
 }
 window.getDecodeHistoryRetentionMs = currentDecodeHistoryRetentionMs;
 window.applyDecodeHistoryRetention = function() {
-  for (const decoder of ["aprs", "hf_aprs", "ais", "vdes", "ft8", "ft4", "ft2", "wspr", "wefax"]) {
+  for (const decoder of ["aprs", "hf_aprs", "ais", "vdes", "ft8", "ft4", "ft2", "wspr", "wefax", "sstv"]) {
     window.trxPluginRuntime.prune(decoder);
   }
 };
@@ -4820,6 +4821,9 @@ function render(update) {
   for (const [key, entry] of Object.entries(_decoderToggles)) {
     syncDecoderToggle(entry, !!update[key], entry.label);
   }
+  if (typeof update.sstv_decode_enabled === "boolean" && window.syncSstvToggle) {
+    window.syncSstvToggle(update.sstv_decode_enabled);
+  }
   if (typeof update.wefax_decode_enabled === "boolean" && window.syncWefaxToggle) {
     window.syncWefaxToggle(update.wefax_decode_enabled);
   }
@@ -7594,9 +7598,17 @@ function updateDecodeStatus(text) {
     if (el && el.textContent !== "Receiving") el.textContent = text;
   }
 }
+var IMAGE_DECODE_KINDS = /* @__PURE__ */ new Set([
+  "lrpt_image",
+  "lrpt_progress",
+  "wefax",
+  "wefax_progress",
+  "sstv",
+  "sstv_progress"
+]);
 function dispatchDecodeMessage(msg, skipStats = false) {
   if (msg.type) window.trxPluginRuntime.dispatch(msg.type, msg);
-  if (!skipStats && msg.type && msg.type !== "lrpt_image" && msg.type !== "lrpt_progress" && msg.type !== "wefax" && msg.type !== "wefax_progress") {
+  if (!skipStats && msg.type && !IMAGE_DECODE_KINDS.has(msg.type)) {
     window.trx.modules.map?.statsRecordDecode(msg.type, msg.rig_id || msg.remote || null);
     window.trx.modules.map?.scheduleStatsRender();
   }
@@ -7642,7 +7654,7 @@ function loadDecodeHistoryOnMainThread(onReady, onError) {
 }
 function restoreDecodeHistoryGroup(kind, messages) {
   if (!Array.isArray(messages) || messages.length === 0) return;
-  if (kind !== "lrpt_image" && kind !== "lrpt_progress" && kind !== "wefax" && kind !== "wefax_progress") {
+  if (!IMAGE_DECODE_KINDS.has(kind)) {
     for (const msg of messages) {
       window.trx.modules.map?.statsRecordDecode(kind, msg.rig_id || msg.remote || null, msg.ts_ms || void 0);
     }