Add Set Where command

New modal-based command that bulk-sets cell values conditionally:
- Match Column: autocomplete column name picker
- Match Values: textarea with one value per line
- Set Value: the value to write

Scans all rows; where the Match Column cell equals any Match Value,
sets the cell at the cursor's current column to Set Value.

Modal supports full keyboard navigation: Tab between fields,
Enter in Set Value confirms, Escape closes, arrow keys for
autocomplete selection.

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

frontend/index.html

@@ -39,6 +39,24 @@
                 <button id="sort-confirm-btn" class="modal-btn modal-btn-primary">Sort</button>
             </div>
         </div>
+        <div id="set-where-modal" class="hidden">
+            <div class="modal-title">Set Where</div>
+            <div class="modal-body">
+                <label for="sw-match-col">Match Column:</label>
+                <div class="sw-input-wrap">
+                    <input type="text" id="sw-match-col" placeholder="Column name" autocomplete="off" />
+                    <div id="sw-col-autocomplete" class="hidden"></div>
+                </div>
+                <label for="sw-match-values" style="margin-top:10px">Match Values (one per line):</label>
+                <textarea id="sw-match-values" rows="6" placeholder="value1&#10;value2&#10;value3"></textarea>
+                <label for="sw-set-value" style="margin-top:10px">Set Value:</label>
+                <input type="text" id="sw-set-value" placeholder="Value to set" />
+            </div>
+            <div class="modal-actions">
+                <button id="sw-cancel-btn" class="modal-btn">Cancel</button>
+                <button id="sw-confirm-btn" class="modal-btn modal-btn-primary">Apply</button>
+            </div>
+        </div>
         <div id="overlay" class="hidden"></div>
     </div>
     <script src="./src/main.js" type="module"></script>