chat: peer-switch strip, back button moved to flyout header

Switching conversations used to require bouncing back to the list view
and picking from there. Now the thread view has a horizontal peer
strip at the top: one click hops to another conversation, active tab
is highlighted and auto-scrolls into view, unread peers show a dot.
Scrollbar is hidden — vertical wheel delta is redirected to horizontal
scroll so the user can slide the strip with the mouse wheel when it
overflows.

In-thread header is gone; the back button moved next to "chat" in the
flyout header and only shows while a thread is open. Peer name stays
visible via the active tab.

Bumps to 1.9.3.

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', '1.9.2');
+define('APP_VERSION', '1.9.3');
 
 // Database path — writable directory, outside webroot
 // DB_PATH / UPLOAD_DIR can be pre-defined by the Electron router for app isolation