[feat](trx-frontend-http): lay the map filters out as a bar across the top

The filters were a 30rem column parked in the bottom-right corner, covering
a third of the map they filter. Lay them out horizontally instead: one row
per group -- label beside its control, thin rules between -- across the top
of the map, spanning ~87% of its width at 1600px and wrapping to a second
row as it narrows.

It starts clear of Leaflet's zoom buttons and stops short of the corner
controls, which stay outside it: the button that hides the filters cannot
live inside the thing it hides. The bottom-left band legend keeps its place.
The sentence explaining the two path toggles would have swallowed the bar,
so it moves to their tooltips and is shown inline only in the stacked
narrow-screen layout.

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

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -1015,7 +1015,7 @@ SPDX-License-Identifier: GPL-2.0-or-later
               <option value="">All</option>
             </select>
           </div>
-          <div class="map-locator-filter-group">
+          <div class="map-locator-filter-group map-filter-grow">
             <span class="map-locator-filter-label">Search</span>
             <input type="text" id="map-search-filter" class="map-search-input" placeholder="Callsign, MMSI, locator, message..." />
           </div>
@@ -1034,9 +1034,9 @@ SPDX-License-Identifier: GPL-2.0-or-later
           <div class="map-locator-filter-group">
             <span class="map-locator-filter-label">Paths</span>
             <div class="map-locator-phase-row">
-              <button type="button" id="map-p2p-paths-toggle" class="map-locator-phase-btn">TRX Paths On</button>
-              <button type="button" id="map-contact-paths-toggle" class="map-locator-phase-btn">Contact Paths On</button>
-              <span class="map-locator-empty">TRX paths on popup, directed decode paths when target locator is known</span>
+              <button type="button" id="map-p2p-paths-toggle" class="map-locator-phase-btn" title="TRX paths are drawn from a station popup">TRX Paths On</button>
+              <button type="button" id="map-contact-paths-toggle" class="map-locator-phase-btn" title="Directed decode paths are drawn when the target locator is known">Contact Paths On</button>
+              <span class="map-paths-hint">TRX paths on popup, directed decode paths when target locator is known</span>
             </div>
           </div>
         </div>