[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/index.html

@@ -584,6 +584,7 @@ SPDX-License-Identifier: GPL-2.0-or-later
         <button class="sub-tab" data-subtab="rds">RDS</button>
         <button class="sub-tab" data-subtab="sat">SAT</button>
         <button class="sub-tab" data-subtab="wefax">WEFAX</button>
+        <button class="sub-tab" data-subtab="sstv">SSTV</button>
       </div>
       <div id="subtab-overview" class="sub-tab-panel">
         <div class="plugin-item" data-decoder="ais">
@@ -646,6 +647,12 @@ SPDX-License-Identifier: GPL-2.0-or-later
             Decodes Meteor-M LRPT (137 MHz QPSK) weather satellite imagery.
           </div>
         </div>
+        <div class="plugin-item" data-decoder="sstv">
+          <strong>SSTV Decoder</strong>
+          <div style="color:var(--text-muted); font-size:0.85rem; margin-top:0.2rem;">
+            Slow-Scan Television &mdash; pictures in Martin, Scottie, Robot, PD and Wraase modes
+          </div>
+        </div>
         <div class="plugin-item" data-decoder="wefax">
           <strong>WEFAX Decoder</strong>
           <div style="color:var(--text-muted); font-size:0.85rem; margin-top:0.2rem;">
@@ -653,6 +660,56 @@ SPDX-License-Identifier: GPL-2.0-or-later
           </div>
         </div>
       </div>
+      <div id="subtab-sstv" class="sub-tab-panel" style="display:none;">
+        <div class="ft8-controls">
+          <button id="sstv-decode-toggle-btn" type="button">Enable SSTV</button>
+          <small id="sstv-status" style="color:var(--text-muted);">Idle</small>
+        </div>
+        <!-- View selector -->
+        <div class="sat-view-bar">
+          <button id="sstv-view-live" class="sat-view-btn sat-view-active" type="button">Live</button>
+          <button id="sstv-view-history" class="sat-view-btn" type="button">History</button>
+        </div>
+        <!-- Live view -->
+        <div id="sstv-live-view">
+          <div style="margin:0 0 0.5rem;">
+            <div style="color:var(--text-muted); font-size:0.82rem; line-height:1.5;">
+              <strong>Slow-Scan Television</strong> &mdash; pictures sent as tones, in Martin,
+              Scottie, Robot, PD and Wraase modes. Tune a sideband or FM signal and enable the
+              decoder; the mode is read from the header the sender opens with.
+            </div>
+          </div>
+          <div id="sstv-live-container" style="display:none; margin:0.5rem 0;">
+            <div style="display:flex; align-items:center; gap:0.5rem; margin-bottom:0.3rem;">
+              <strong>Receiving</strong>
+              <small id="sstv-live-info" style="color:var(--text-muted);"></small>
+            </div>
+            <canvas id="sstv-live-canvas" width="320" height="256"
+                    style="width:100%; max-width:640px; image-rendering:pixelated; background:#111;"></canvas>
+          </div>
+          <div id="sstv-live-latest" style="margin-top:0.5rem;"></div>
+        </div>
+        <!-- History view -->
+        <div id="sstv-history-view" style="display:none;">
+          <div class="sat-history-controls">
+            <input id="sstv-filter" class="ft8-filter" type="text" placeholder="Filter (e.g. Martin, PD120)" />
+            <select id="sstv-sort" class="sat-sort-select">
+              <option value="newest">Newest first</option>
+              <option value="oldest">Oldest first</option>
+            </select>
+            <button id="sstv-clear-btn" type="button" style="font-size:0.8rem;">Clear All</button>
+          </div>
+          <div class="sat-history-header">
+            <span class="sat-col-time">Time</span>
+            <span class="sat-col-type">Mode</span>
+            <span class="sat-col-sat">Size</span>
+            <span class="sat-col-lines">Lines</span>
+            <span class="sat-col-link">Image</span>
+          </div>
+          <div id="sstv-history-list"></div>
+          <small id="sstv-history-count" style="color:var(--text-muted);font-size:0.75rem;">No pictures yet</small>
+        </div>
+      </div>
       <div id="subtab-rds" class="sub-tab-panel" style="display:none;">
         <div class="rds-grid">
           <div class="rds-field"><span class="rds-label">Status</span><span id="rds-status" class="rds-value rds-no-signal">No signal</span></div>