Replace mode toggle cycle with a dropdown menu

The composite mode control (#planModeBtn) now opens a dropdown menu
instead of cycling, following the emoji-picker pattern: each mode is an
entry with icon + label + description, the active one marked with a
check. Direct jumps between modes are now possible, so selecting
plan/ultracode resets the persisted panel mode to avoid running both the
panel and the CLI worker. Unified the active button states.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wwwroot/index.html

@@ -458,7 +458,7 @@
         <input type="file" id="fileInput" accept="image/jpeg,image/png,image/gif,image/webp,application/pdf" multiple hidden>
         <button id="fileBtn" class="icon-btn" title="Datei anhängen"><i class="fa fa-paperclip"></i></button>
         <button id="explorerBtn" class="icon-btn" title="Workspace durchsuchen"><i class="fa fa-folder"></i></button>
-        <button id="planModeBtn" class="icon-btn" title="Plan-Modus / Panel umschalten"><i class="fa fa-map"></i></button>
+        <button id="planModeBtn" class="icon-btn" title="Modus wählen"><i class="fa fa-ban"></i></button>
         <textarea id="input" placeholder="Nachricht..." rows="1"></textarea>
         <button id="emojiBtn" class="icon-btn" title="Emoji einfügen"><i class="fa fa-smile-o"></i></button>
         <button id="send" class="icon-btn"><i class="fa fa-paper-plane"></i></button>