[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>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -1345,7 +1345,10 @@ small { color: var(--text-muted); }
 .top-bar-actions > * {
   flex: 0 0 auto;
 }
-.header-bar-btn.header-audio-btn {
+/* Square icon buttons in the top bar: an icon with no box of its own has no
+   size to draw at, so the button gives it one. */
+.header-bar-btn.header-audio-btn,
+.header-bar-btn.header-share-btn {
   width: 2rem;
   height: 2rem;
   min-height: 0;
@@ -1360,10 +1363,15 @@ small { color: var(--text-muted); }
   cursor: pointer;
   flex-shrink: 0;
 }
-.header-audio-btn svg {
+.header-audio-btn svg,
+.header-share-btn svg {
   width: 100%;
   height: 100%;
 }
+/* The link icon is drawn in strokes, not fills, and wants room around it. */
+.header-share-btn svg {
+  padding: 1px;
+}
 .header-audio-btn.audio-active {
   color: #00d17f;
   border-color: #00d17f;