chat: merge caret + title into one back trigger

Caret and "chat" label were two hit targets with two hover states and
tooltips. Collapse them into a single <button id="chat-header-nav">
with one tooltip ("back to conversations") — the caret becomes a
visibility-only hint inside the button, shown when there's somewhere
to go back to, hidden on the list itself. The .navigable class
controls whether the button reads as interactive, so clicking the
label while already on the list is a cursor-default no-op.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config.php

@@ -1,7 +1,7 @@
 <?php
 
 define('APP_NAME', 'hyprcrd');
-define('APP_VERSION', '1.9.11');
+define('APP_VERSION', '1.9.12');
 
 // Database path — writable directory, outside webroot
 // DB_PATH / UPLOAD_DIR can be pre-defined by the Electron router for app isolation