fix(workbar): keep scroll arrows clear of tab close buttons

Reserve space on both sides of the tab viewport for overflow arrows so they cannot cover tab close controls. Keep that space while arrows are hidden or disabled to avoid layout shifts.

Add Electron coverage for arrow bounds and native closing at the right viewport edge; update tab layout documentation.

Validation: 19 focused tests passed; typecheck and lint passed. Full suite did not complete before interruption and reported failures in supervisor settings and supervisor layout tests.

Release note: Fix workbar scroll arrows covering tab close buttons when many tabs are open.

src/renderer/src/styles.css

@@ -5055,9 +5055,9 @@ button > svg {
 }
 
 .workbar-shell__scroll-button {
-  position: absolute;
-  z-index: 2;
-  bottom: 5px;
+  flex: 0 0 28px;
+  align-self: flex-end;
+  margin-bottom: 5px;
   background: transparent;
   opacity: 0;
   pointer-events: none;
@@ -5070,14 +5070,6 @@ button > svg {
   pointer-events: auto;
 }
 
-.workbar-shell__scroll-button:first-child {
-  left: 0;
-}
-
-.workbar-shell__scroll-button:last-child {
-  right: 0;
-}
-
 .workbar-shell__scroll-button:hover:not(:disabled) {
   background: var(--surface-muted);
   color: var(--text-primary);