ui: draw the sidebar tabs without the placeholder icon

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -2909,7 +2909,7 @@ now — they are listed so they are not mistaken for anything else:
   unlocked, because configuring the host is what a first run is for. Each pane's heading also carries its **own switch**, the same control as the
   header bar's button — clicking either moves both — because a pane that is *disabled* while its
   feature is off is a pane whose controls you would otherwise have to leave to turn it back on. The
-  switcher's policy is `wide` and each page has a title. With the math
+  switcher's policy is `wide` and each page has a title and no icon (the switcher's own placeholder icon is hidden). With the math
   list folded into the signal list under a `MATH` divider whose `+` creates a channel — a derived channel already *is* a channel in
   the model, so the two lists were the same list drawn twice. A tab is one-of-*N*, so no
   toggle hides a pane any more: cursors-off **disables** its readout and leaves it in place, and