Lay out Available Mults and Qs like N1MM's window

The window is now a summary table of mults, Qs and totals per band over a
sortable spot grid with Call, Freq, Mode, Dir, Mlt, SN, TS, Spotter, Grid,
Sun, Pts and a preferred-spotter column. Columns can be hidden, headers
sorted, and the list stops re-sorting for six seconds under the mouse.

Adds the Mults / Mults & Qs button, a Bands & Modes filter, delete and
blacklist from the right-click menu, Alt+A stepping, and QSY to the top spot
after logging. Spot gains Snr and IsSkimmer, Bandmap gains Remaining, and
AvailableStation carries the path to the station.

The menu item no longer has apostrophes. SO2R, SO2V and the other missing
pieces are listed in docs/unfinished.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.Spotting/Bandmap.cs

@@ -106,6 +106,19 @@ public sealed class Bandmap
         }
     }
 
+    /// How long the spot has left before it drops off the map, or null for a
+    /// spot that is not on it.
+    public TimeSpan? Remaining(Spot spot)
+    {
+        string key = Key(spot);
+        lock (guard)
+        {
+            return arrived.TryGetValue(key, out DateTime at)
+                ? (LifetimeFor?.Invoke(spot) ?? Lifetime) - (clock() - at)
+                : null;
+        }
+    }
+
     public IReadOnlyList<Spot> On(Band band)
     {
         lock (guard)