[feat](trx-frontend-http): put the tuned frequency in the address bar

A receiver spreads by being linked to, and there was nothing to link to:
the routes carried the tab and nothing else, so "listen to this" could
only ever mean a screenshot and a frequency typed out in a message.

The query string now carries the dial -- rig, frequency, mode and
bandwidth -- in both directions.  Opening a link selects the rig, sets
the mode, tunes, then applies the bandwidth: a mode change brings its
own default bandwidth with it, so an explicit bw has to land after it.
Frequencies are read the way someone writes them by hand (7074k,
14.074M) and written back as whole Hz, so what comes out of the address
bar is the same link in canonical form.

After that the address bar keeps up with the dial, which is what makes
it copyable at any moment rather than only at load.  It is rewritten
with replaceState -- tuning is not navigation, and a swept dial would
otherwise bury the back button.  A link button in the top bar copies
the current link; it folds into the overflow menu when the bar is tight.

Applying a link changes the radio, so an rx session says so instead of
failing control calls one at a time.  A tab listening to a virtual
channel leaves the address alone rather than publishing a frequency the
rig is not on, and bw is skipped in both directions on rigs without
filter control, which would only refuse it.

The fixture pinned every state frame to 100 MHz plus jitter to keep
frames distinct, so no test could observe tuning at all.  The jitter
moves to the S-meter and the fixture echoes set_freq/set_mode/
set_bandwidth, as it already did for squelch.

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

docs/User-Manual.md

@@ -300,6 +300,39 @@ loopback on Linux, BlackHole on macOS).
 
 ---
 
+## Tune Links
+
+Every page of the web UI carries what the radio is doing in its address, so the
+URL in the address bar is always a link someone else can open:
+
+```
+http://receiver.example:8080/?rig=sdr&f=14074000&mode=USB&bw=3000
+```
+
+| Parameter | Meaning |
+|-----------|---------|
+| `f` | Frequency. Hz by default; `7074k` and `14.074M` also work. |
+| `mode` | Demodulation mode, e.g. `USB`, `CW`, `WFM`. |
+| `bw` | Filter bandwidth in Hz. Ignored by rigs without filter control. |
+| `rig` | Rig to select first, by id, on a multi-rig client. |
+
+Opening such a link selects the rig, sets the mode, tunes, and applies the
+bandwidth, in that order — a mode change carries its own default bandwidth, so
+an explicit `bw` is applied last. Anything the rig cannot do (an unknown mode,
+a frequency outside its range) is reported and the rest of the link still
+applies. All four parameters are optional.
+
+The link button in the top bar copies the current link to the clipboard. The
+address bar itself is updated as you tune, using `replaceState`, so sweeping
+the dial does not fill the browser's history.
+
+Applying a link changes the radio, so it needs the `control` role; an `rx`
+session opens the page and says the link was not applied. Links describe the
+rig's own dial — while a tab is listening to a virtual channel the address is
+left as it was, rather than publishing a frequency the rig is not on.
+
+---
+
 ## Authentication
 
 The HTTP frontend supports optional passphrase-based authentication with two