feat(dvk): twelve slots; perf: Settings pauses the heavy streams

The voice keyer grows to F1-F12 — the UI was already data-driven, so the
count constant, the F-key range and the labels are the whole change.

And the preferences stop stuttering on a busy station: every spot batch,
decode batch and CAT snapshot re-rendered the entire App tree behind the
dialog — cluster grid, decodes panel, thousands of nodes — and the pointer
janked over the very panel the operator was trying to use. While Settings is
open the flushes park their batches in the pending refs (bounded) and CAT
repaints at most every two seconds; closing the dialog drains everything.

app.go

@@ -10239,7 +10239,7 @@ func titleEntity(s string) string {
 // the configured "Recording mic", transmit via "To Radio", preview via
 // "Listening".
 
-const dvkSlots = 6
+const dvkSlots = 12
 
 // DVKMessage is one voice-keyer slot for the UI.
 type DVKMessage struct {