[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/frontend/tests/web-fixture.mjs

@@ -29,6 +29,7 @@ const DECODER_REGISTRY = [
   { 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: "sstv", label: "SSTV", activation: "toggle", active_modes: ["USB", "FM"] },
   { 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"] },
@@ -104,7 +105,7 @@ function encodeCbor(value) {
   return Buffer.concat(chunks);
 }
 
-const HISTORY_GROUPS = ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "ft4", "ft2", "wspr", "wefax"];
+const HISTORY_GROUPS = ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "ft4", "ft2", "wspr", "wefax", "sstv"];
 
 function assetPath(urlPath) {
   // Every tab route has its own index handler on the server (see api/assets.rs),
@@ -203,6 +204,7 @@ export async function startWebFixture({
     wspr_decode_enabled: false,
     lrpt_decode_enabled: false,
     wefax_decode_enabled: false,
+    sstv_decode_enabled: false,
     recorder_enabled: false,
     clients: 1,
     rigctl_clients: 0,