Add Sort Advanced command with multi-column sort

Opens a modal with a text field supporting autocomplete of column
names. Enter comma-separated column names in priority order;
rows are sorted ascending using locale-aware numeric comparison,
first by the first column, then by the second, and so on.

Autocomplete filters to matching headers, excludes already-chosen
columns, and supports keyboard navigation (arrows + enter).

Co-authored-by: Shelley <shelley@exe.dev>

frontend/index.html

@@ -25,6 +25,20 @@
             <input type="text" id="command-input" placeholder="Type a command..." />
             <div id="command-list"></div>
         </div>
+        <div id="sort-modal" class="hidden">
+            <div class="modal-title">Sort Advanced</div>
+            <div class="modal-body">
+                <label for="sort-columns-input">Columns (comma-separated, in priority order):</label>
+                <div id="sort-input-wrap">
+                    <input type="text" id="sort-columns-input" placeholder="e.g. Name, Age, City" autocomplete="off" />
+                    <div id="sort-autocomplete" class="hidden"></div>
+                </div>
+            </div>
+            <div class="modal-actions">
+                <button id="sort-cancel-btn" class="modal-btn">Cancel</button>
+                <button id="sort-confirm-btn" class="modal-btn modal-btn-primary">Sort</button>
+            </div>
+        </div>
         <div id="overlay" class="hidden"></div>
     </div>
     <script src="./src/main.js" type="module"></script>