refactor: ui.js, chat.js, history.js in 13 modulare Dateien aufteilen

Die monolithische ui.js (1534 Zeilen) zusammen mit chat.js (612) und
history.js (626) in fokussierte Module aufgeteilt:

Neue Dateien aus ui.js:
- ui-helpers.js (126) - DOM-Utilities, $, esc, truncate, scrollDown, Toast
- markdown.js (143) - renderMarkdown, wrapKreaGalleries, LaTeX/Code-Schutz
- cards.js (90) - buildSessionCard, buildAgentCard
- msg-actions.js (208) - Delete, Mark, Share, Reply-Button, Timestamp
- tool-ui.js (332) - Tool-Boxes, Diffs, Todo-Liste, Visibility
- modals.js (190) - System-Prompt, Command-Output, Krea-Image Modals
- stream-ui.js (124) - Stream-Bubble, Review-Streaming
- bubbles.js (312) - User/Assistant/Error/Warning/Choice/Review Bubbles

Neue Dateien aus chat.js:
- reply.js (54) - Reply/Quote-Handling mit pendingReply State
- files.js (110) - File-Upload, Workspace-Refs mit pendingFiles/Refs State

Neue Datei aus history.js:
- archive.js (173) - Archiv-Loading mit loadedArchiveCount State

Verbleibend:
- chat.js (448) - Stream-Events, sendMessage, handleChoiceClick, Plan-Mode
- history.js (464) - History laden, Polling, Message-Rendering

Kein Code verändert, nur verschoben. Alle 94 Funktionen verifiziert.
Globale Variablen in die Module verschoben, wo sie primär genutzt werden.

wwwroot/index.html

@@ -555,12 +555,22 @@
 <script src="js/tooltip.js"></script>
 <script src="js/emoji.js"></script>
 <script src="js/emoji.userdata.js"></script>
-<script src="js/ui.js"></script>
+<script src="js/ui-helpers.js"></script>
+<script src="js/markdown.js"></script>
+<script src="js/cards.js"></script>
+<script src="js/msg-actions.js"></script>
+<script src="js/tool-ui.js"></script>
+<script src="js/modals.js"></script>
+<script src="js/stream-ui.js"></script>
+<script src="js/bubbles.js"></script>
 <script src="js/api.js"></script>
 <script src="js/session.js"></script>
+<script src="js/reply.js"></script>
+<script src="js/files.js"></script>
 <script src="js/file-explorer.js"></script>
 <script src="js/chat.js"></script>
 <script src="js/history.js"></script>
+<script src="js/archive.js"></script>
 <script src="js/router.js"></script>
 <script src="js/commands-ui.js"></script>
 <script src="js/avatar-editor.js"></script>