feat: add case reorder and delete in Manage tab

Add a "Manage" tab to the create-case modal with up/down reorder
buttons and delete for each case. Linked cases are unlinked (folder
preserved); CASES_DIR cases are permanently deleted.

Backend:
- DELETE /api/cases/:name — unlink or delete
- PUT /api/cases/order — persist ordering to settings.json
- GET /api/cases now respects saved caseOrder

Frontend:
- Third "Manage" tab in createCaseModal with case list
- Delete button in mobile case picker bottom sheet
- SSE events: case:deleted, case:order-changed

src/web/public/index.html

@@ -1360,6 +1360,7 @@
         <div class="modal-tabs">
           <button class="modal-tab-btn active" data-tab="case-create">Create New</button>
           <button class="modal-tab-btn" data-tab="case-link">Link Existing</button>
+          <button class="modal-tab-btn" data-tab="case-manage">Manage</button>
         </div>
         <div class="modal-body">
           <!-- Create New Tab -->
@@ -1387,6 +1388,13 @@
               <span class="form-hint">Absolute path to an existing project folder, e.g. /home/you/my-project</span>
             </div>
           </div>
+          <!-- Manage Tab -->
+          <div class="modal-tab-content hidden" id="case-manage">
+            <div class="case-manage-list" id="caseManageList">
+              <!-- Populated by JS -->
+            </div>
+            <span class="form-hint" style="margin-top: 8px; display: block;">Drag or use arrows to reorder. Changes are saved automatically.</span>
+          </div>
         </div>
         <div class="form-actions">
           <button class="btn-toolbar" onclick="app.closeCreateCaseModal()">Cancel</button>