bar: fix fullscreen-detection race during cross-monitor focus moves (#2373)

hasFullscreenToplevelOnScreen's fast-path used `NiriService.currentOutput
=== screenName` as a proxy for "the active toplevel is on screenName".
Those are two independent state channels (niri's workspace-activated
events drive currentOutput; wlr-foreign-toplevel-management drives
ToplevelManager.activeToplevel) and they don't update atomically.

When focus crosses from a fullscreen toplevel on monitor A to a
non-fullscreen toplevel on monitor B, currentOutput flips to B before
activeToplevel updates away from A's still-fullscreen-and-activated
window. For one tick, B's bar evaluates `active.fullscreen &&
active.activated && currentOutput === B` → true, hides itself, then
flips back when activeToplevel finally updates. Visible as a one-frame
bar flicker on focus changes between monitors when one has a fullscreen
window.

Replace the proxy with _toplevelOnScreen(active, screenName), the same
helper the X11 fallback path uses 25 lines below. The check now
inspects the toplevel's actual outputs instead of trusting a separate
state signal, so the race can't fire.

The per-workspace loop below was already correct; it would catch any
real fullscreen+activated toplevel on the bar's workspace regardless
of focused output. The fast-path was redundant when the assertion
held and wrong when it didn't.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

quickshell/Services/CompositorService.qml

@@ -417,7 +417,7 @@ Singleton {
 
         if (isNiri) {
             const active = ToplevelManager.activeToplevel;
-            if (active?.fullscreen && active?.activated && NiriService.currentOutput === screenName)
+            if (active?.fullscreen && active?.activated && _toplevelOnScreen(active, screenName))
                 return true;
 
             const filtered = filterCurrentWorkspace(sortedToplevels, screenName);