feat: customizable delete session command template

Symmetric with create template. Default: 'tmux kill-session -t {name}'.
Configurable in Settings → Commands tab (renamed from 'New Session').
DELETE /api/sessions/{name} reads template from settings, substitutes
{name}, runs synchronously via subprocess.run with 30s timeout.

Enables custom teardown workflows like:
  amplifier-dev ~/dev/{name} --destroy

Changes:
- settings.py: add delete_session_template to DEFAULT_SETTINGS
- main.py: DELETE endpoint uses subprocess.run + template instead of run_tmux
- index.html: add delete template textarea + reset btn; rename tab to Commands
- app.js: DELETE_SESSION_DEFAULT_TEMPLATE constant, openSettings loads it,
  bindStaticEventListeners wires input/reset; constant exported

Tests added:
- test_settings.py: 3 tests for default, load, and patch of delete_session_template
- test_api.py: 2 tests for custom/default template substitution via subprocess mock;
  update existing test_delete_session_calls_kill_session to match new implementation
- test_frontend_html.py: 5 tests for textarea, placeholder, rows, reset btn, tab label
- test_app.mjs: 5 tests for constant definition, value, and wiring in openSettings/bindStaticEventListeners

muxplex/frontend/index.html

@@ -87,7 +87,7 @@
         <button class="settings-tab settings-tab--active" data-tab="display">Display</button>
         <button class="settings-tab" data-tab="sessions">Sessions</button>
         <button class="settings-tab" data-tab="notifications">Notifications</button>
-        <button class="settings-tab" data-tab="new-session">New Session</button>
+        <button class="settings-tab" data-tab="new-session">Commands</button>
       </nav>
       <div class="settings-content">
         <div class="settings-panel" data-tab="display">
@@ -169,6 +169,12 @@
             <span class="settings-helper">{name} is replaced with the session name</span>
             <button id="setting-template-reset" class="settings-action-btn">Reset to default</button>
           </div>
+          <div class="settings-field settings-field--column">
+            <label class="settings-label" for="setting-delete-template">Delete session command</label>
+            <textarea id="setting-delete-template" class="settings-textarea" rows="3" placeholder="tmux kill-session -t {name}"></textarea>
+            <span class="settings-helper">{name} is replaced with the session name</span>
+            <button id="setting-delete-template-reset" class="settings-action-btn">Reset to default</button>
+          </div>
         </div>
       </div>
     </div>