[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-protocol/src/decoders.rs

@@ -139,6 +139,16 @@ pub const DECODER_REGISTRY: &[DecoderDescriptor] = &[
         background_decode: false,
         bookmark_selectable: true,
     },
+    DecoderDescriptor {
+        id: "sstv",
+        label: "SSTV",
+        activation: DecoderActivation::Toggle,
+        // SSTV is sent on sideband on HF and on FM simplex on VHF, and the
+        // decoder cares only about the audio it is handed.
+        active_modes: &["USB", "LSB", "FM", "AM", "DIG"],
+        background_decode: false,
+        bookmark_selectable: true,
+    },
 ];
 
 // ============================================================================

src/trx-protocol/src/mapping.rs

@@ -122,7 +122,8 @@ define_command_mapping! {
         ResetFt2Decoder      <=> ResetFt2Decoder,
         ResetWsprDecoder     <=> ResetWsprDecoder,
         ResetLrptDecoder     <=> ResetLrptDecoder,
-        ResetWefaxDecoder    <=> ResetWefaxDecoder;
+        ResetWefaxDecoder    <=> ResetWefaxDecoder,
+        ResetSstvDecoder     <=> ResetSstvDecoder;
 
     // ── Single-field struct <=> tuple ────────────────────────────────
     field:
@@ -140,6 +141,7 @@ define_command_mapping! {
         SetWsprDecodeEnabled  { enabled }        <=> SetWsprDecodeEnabled,
         SetLrptDecodeEnabled  { enabled }        <=> SetLrptDecodeEnabled,
         SetWefaxDecodeEnabled { enabled }        <=> SetWefaxDecodeEnabled,
+        SetSstvDecodeEnabled  { enabled }        <=> SetSstvDecodeEnabled,
         SetBandwidth          { bandwidth_hz }   <=> SetBandwidth,
         SetSdrGain            { gain_db }        <=> SetSdrGain,
         SetSdrLnaGain         { gain_db }        <=> SetSdrLnaGain,

src/trx-protocol/src/types.rs

@@ -73,6 +73,9 @@ pub enum ClientCommand {
     SetWefaxDecodeEnabled {
         enabled: bool,
     },
+    SetSstvDecodeEnabled {
+        enabled: bool,
+    },
     ResetAprsDecoder,
     ResetHfAprsDecoder,
     ResetCwDecoder,
@@ -82,6 +85,7 @@ pub enum ClientCommand {
     ResetWsprDecoder,
     ResetLrptDecoder,
     ResetWefaxDecoder,
+    ResetSstvDecoder,
     SetBandwidth {
         bandwidth_hz: u32,
     },