Tighten Sort Unsorted matching; consolidate file management tools

Fix Sort Unsorted picking a wrong destination on words that are common
clutter across many categories (a creator's name, generic marketing
filler) by ranking matches on a folder's own deliberate name over
incidental sibling-course words, and downweighting borrowed words that
recur across many categories. Add a folder-picker/create-new/rename UI
to override any proposal.

Turn /unsorted into a general File Management page: move Refresh
Library, Bulk Rename, and Manage Library (hide/rename) here from
Settings, add a Move action to Manage Library using the same picker,
and add Duplicate Courses and Clean Up Stale References tools. Add a
footer link to the page from the dashboard.

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

templates/course_dashboard.html

@@ -197,6 +197,21 @@
             transform: translateY(0);
         }
 
+        .page-footer-link {
+            text-align: center;
+            padding: 24px 0 8px;
+        }
+        .page-footer-link a {
+            color: var(--text-muted);
+            text-decoration: none;
+            display: inline-flex;
+            align-items: center;
+            gap: 6px;
+            font-size: 0.85em;
+        }
+        .page-footer-link a:hover {
+            color: var(--accent);
+        }
         .bottom-tab-bar {
             position: fixed;
             left: 0;
@@ -1313,6 +1328,10 @@
         </div>
         {% endif %}
     {% endif %}
+
+    <div class="page-footer-link">
+        <a href="/unsorted">{{ icons.icon('folder', 14) }} File Management</a>
+    </div>
     </div>
 
     <nav class="bottom-tab-bar">