Add 'vaultik remote nuke', rename Processing→Backing up, bits/sec rates

remote nuke: new subcommand that deletes every snapshot's metadata and
every blob from remote storage, leaving the bucket prefix empty.
Requires --force.

User-facing 'Processing' is now 'Backing up' everywhere it referred to
the chunking/upload phase. Files summary line says 'backed up' instead
of 'processed'.

ui.Speed now formats bytes/sec input as bits/sec output (bit/s, Kbit/s,
Mbit/s, Gbit/s). Network transfer rates are conventionally expressed
in bits — the per-blob heartbeat now matches the per-snapshot summary
line which has always been bits/sec.

internal/ui/ui_test.go

@@ -100,6 +100,17 @@ func TestValueFormattersPlain(t *testing.T) {
 		t.Errorf("Percent: got %q", got)
 	}
 
+	// Speed: input is bytes/sec, output is bits/sec.
+	if got := w.Speed(0); got != "N/A" {
+		t.Errorf("Speed(0): got %q, want N/A", got)
+	}
+	if got := w.Speed(125_000_000); got != "1.0 Gbit/sec" { // 1 Gbit/s = 125 MB/s
+		t.Errorf("Speed(125e6): got %q", got)
+	}
+	if got := w.Speed(125_000); got != "1 Mbit/sec" {
+		t.Errorf("Speed(125e3): got %q", got)
+	}
+
 	// Time format: today → HH:MM:SS, other day → YYYY-MM-DD HH:MM:SS.
 	today := time.Date(time.Now().Year(), time.Now().Month(), time.Now().Day(), 14, 30, 45, 0, time.Local)
 	if got := w.Time(today); got != "14:30:45" {