[fix](trx-frontend-http): show one rig at a time on the digital modes page

A client connected to several rigs decodes all of them at once, and the
decode stream carries every rig's traffic to every browser.  The decoder
panels listed all of it: a station a background rig copied on another band
appeared in the APRS list next to the selected rig's, the vessel counts and
the "latest seen" lines counted both, the status lines said "Receiving"
because some other rig was, and the CW pane interleaved two rigs into one
stream of text that read as neither.

The page is about the rig the operator selected — the one whose spectrum is
on screen and whose audio is playing — so each panel now shows what that rig
heard: rows, counts, latest-seen, status, the live picture a WEFAX or SSTV
frame is painting, and the CW pane.

Nothing is dropped on the way in.  The map is the whole station's view, has
its own rig filter, and would empty out if the plugins stopped feeding it, so
the histories still hold every rig and the map still plots them.  That also
means a switch loses nothing: the runtime gained a rerender hook, which the
rig switch calls, and switching back brings the other rig's traffic up again.
The CW pane is the exception — a running stream of text cannot be unpicked
after the fact — so it starts empty on the rig switched to.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

docs/User-Manual.md

@@ -454,6 +454,29 @@ listener and every rig's audio listener.
 
 ---
 
+## Multiple Rigs in the Web UI
+
+A client connected to several rigs decodes all of them at once, whichever one
+is on screen. The rig picker in the header decides which rig the UI is about,
+and each page answers that differently:
+
+| Page | Shows |
+|------|-------|
+| Radio | The selected rig: its spectrum, its audio, and the mini decode views over the waterfall. |
+| Digital Modes | The selected rig: every decoder panel, its counts and its status line. |
+| Map | The whole station — every rig's positions, with the map's own rig filter to narrow it. |
+| Statistics | The whole station, including the per-rig comparison. |
+
+Switching rigs repaints the radio and digital modes pages for the rig now
+selected. Nothing is lost by switching: the traffic other rigs heard is still
+held, and switching back brings it up again. The one exception is the CW pane,
+which is a single running stream of copied text rather than a list of frames,
+so it starts empty on the rig you switch to.
+
+Each browser tab keeps its own selection, so two tabs can watch two rigs.
+
+---
+
 ## Tune Links
 
 Every page of the web UI carries what the radio is doing in its address, so the