Fix sidebar collapse flash on page navigation

Suppress CSS transitions during initial state restore from localStorage.
Re-enable after two rAF frames so the toggle button still animates smoothly.

Co-authored-by: Copilot <[email protected]>

static/js/theme.js

@@ -17,8 +17,18 @@
         }
     }
 
-    // Restore saved state
+    // Restore saved state instantly — suppress transitions so there's no
+    // collapse animation flash on every page navigation.
+    sidebar.style.transition = 'none';
+    contentWrapper.style.transition = 'none';
     applyCollapsed(localStorage.getItem(COLLAPSED_KEY) === '1');
+    // Re-enable transitions after one frame so the toggle button still animates.
+    requestAnimationFrame(function () {
+        requestAnimationFrame(function () {
+            sidebar.style.transition = '';
+            contentWrapper.style.transition = '';
+        });
+    });
 
     if (toggleBtn) {
         toggleBtn.addEventListener('click', function () {