fix focustop(): prefer a visible fullscreen client

Problem (reproducible on vanilla 0.9, unrelated to any local patch):
1. fullscreen client A on tag 3 (MOD+e);
2. MOD+0 to view all tags;
3. MOD+1 to switch to tag 1, focus lands on client B;
4. MOD+0 again: client A is displayed fullscreen, covering everything,
   but the focused client is the invisible B;
5. so MOD+e / MOD+q act on B behind A and the user thinks MOD+e does
   not work on A until A is clicked with the mouse.

Root cause: focustop() returns the first VISIBLEON() client on the
fstack, but VISIBLEON() only checks whether the client is on a
currently visible tag; it does not check whether the client is covered
by a fullscreen client. When switching back to tag 0, the last focused
client B is first on the fstack, so B gets the focus while A is still
displayed fullscreen.

Fix: focustop() first looks for a visible fullscreen client on the
fstack (at most one fullscreen client per monitor is actually shown,
and it always sits on top, covering the whole monitor), and only falls
back to the previous "first visible client" if there is none. As a
result:
- after a tag switch the focus goes back to the fullscreen client that
  actually occupies the screen, so MOD+e / MOD+q behave as expected;
- the fullscreen_bg decision in arrange() is fixed as a side effect
  (it used to be turned off when the focus was not on the fullscreen
  client, so a non-opaque fullscreen surface could reveal other
  clients behind it, which violates the protocol);
- focusstack() already returns early when the focused client is
  fullscreen, so MOD+j/k and the arrow keys no longer move the focus
  to a covered client either.

Verified with headless dwl, two foot clients and keys injected through
zwp_virtual_keyboard_v1: before the fix the last step gives
focal = winB / fullscreen 0; after the fix focal = winA / fullscreen 1,
and MOD+e then restores A as expected.

ta: 20261031_022753

dwl.c

@@ -1700,10 +1700,14 @@ Client *
 focustop(Monitor *m)
 {
 	Client *c;
-	wl_list_for_each(c, &fstack, flink) {
+
+	wl_list_for_each(c, &fstack, flink)
+		if (VISIBLEON(c, m) && c->isfullscreen)
+			return c;
+
+	wl_list_for_each(c, &fstack, flink)
 		if (VISIBLEON(c, m))
 			return c;
-	}
 	return NULL;
 }