Make browser back button step up one Library folder instead of exiting

Drilling into a Library folder only ever swapped #library-groups's
contents via fetch, never touching browser history - so back had
nothing of the app's own to step through once you were a few folders
deep, and fell straight through to exiting the app instead of going up
a level.

Push a history entry per drill-down and breadcrumb jump, and restore
libraryTrail/re-fetch that level on popstate, so back now un-drills
one folder at a time and only reaches the app's actual exit once
you're back at the library root.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/course_dashboard.html

@@ -1133,8 +1133,27 @@
             if (!libraryCard) return;
             libraryTrail = [];
             fetchLibraryLevel(null);
+            history.replaceState({ libraryTrail: [], path: null }, '');
         }
 
+        // Drilling into a folder only ever swapped #library-groups's
+        // contents - no history entry, no URL change - so the back
+        // button/gesture had nothing of ours to step back through once
+        // you were a few folders deep, and fell straight through to
+        // exiting the app instead of going up one level. Pushing a state
+        // per level (and restoring it on popstate, below) makes back
+        // step back up the library trail one folder at a time instead.
+        function pushLibraryState() {
+            history.pushState({ libraryTrail: libraryTrail.slice(), path: libraryTrail.length ? libraryTrail[libraryTrail.length - 1].path : null }, '');
+        }
+
+        window.addEventListener('popstate', function(event) {
+            const state = event.state;
+            if (!state || !document.getElementById('library-card')) return;
+            libraryTrail = state.libraryTrail || [];
+            fetchLibraryLevel(state.path || null);
+        });
+
         function fetchLibraryLevel(path) {
             searchActive = false;
             const searchInput = document.getElementById('library-search-input');
@@ -1176,11 +1195,13 @@
             if (index < 0) {
                 libraryTrail = [];
                 fetchLibraryLevel(null);
+                pushLibraryState();
                 return;
             }
             const target = libraryTrail[index];
             libraryTrail = libraryTrail.slice(0, index + 1);
             fetchLibraryLevel(target.path);
+            pushLibraryState();
         }
 
         function renderLibraryLevel(data) {
@@ -1202,6 +1223,7 @@
         function enterLibraryDir(path, name) {
             libraryTrail.push({ name: name, path: path });
             fetchLibraryLevel(path);
+            pushLibraryState();
         }
 
         // A few shimmering placeholder rows for in-flight fetches, instead