feat: channel management — create, edit, delete, reorder with category-type enforcement

Server:
- Enforce category-type validation: text/announcement only under text categories,
  voice only under voice categories (400 on mismatch)
- Admin panel category field changed to dropdown with auto-filtered type options
- Default setup creates both Text Channels and Voice Channels categories

Client:
- Add create/edit/delete channel modals (admin/owner only)
- "+" button on category headers to create channels with pre-filled category
- Right-click context menu on channels for edit/delete
- Mouse-based drag-and-drop reordering within categories
- Admin API methods: adminCreateChannel, adminUpdateChannel, adminDeleteChannel
- Immediate local store update on reorder for instant feedback

Tests: 7 server integration tests, 31 client unit tests (create/edit/delete modals)

Server/admin/static/index.html

@@ -326,7 +326,12 @@
               <option value="announcement">Announcement</option>
             </select>
           </div>
-          <div class="form-group"><label>Category</label><input id="ch-category" type="text" placeholder="General"></div>
+          <div class="form-group"><label>Category</label>
+            <select id="ch-category">
+              <option value="Text Channels">Text Channels</option>
+              <option value="Voice Channels">Voice Channels</option>
+            </select>
+          </div>
           <div class="form-group"><label>Topic</label><input id="ch-topic" type="text"></div>
           <div class="form-group"><label>Position</label><input id="ch-position" type="number" value="0"></div>
         </div>
@@ -816,6 +821,24 @@ document.getElementById('show-create-channel').onclick = () => {
   document.getElementById('create-channel-form').classList.toggle('hidden');
 };
 
+// Filter channel type dropdown based on selected category
+document.getElementById('ch-category').addEventListener('change', function() {
+  const typeSelect = document.getElementById('ch-type');
+  const isVoice = this.value.toLowerCase().includes('voice');
+  for (const opt of typeSelect.options) {
+    if (isVoice) {
+      opt.hidden = opt.value !== 'voice';
+    } else {
+      opt.hidden = opt.value === 'voice';
+    }
+  }
+  // Auto-select the first visible option
+  const visible = [...typeSelect.options].find(o => !o.hidden);
+  if (visible) typeSelect.value = visible.value;
+});
+// Trigger initial filter
+document.getElementById('ch-category').dispatchEvent(new Event('change'));
+
 document.getElementById('cancel-create-channel').onclick = () => {
   document.getElementById('create-channel-form').classList.add('hidden');
 };