feat: show server version in sidebar header

Version injected at build time via VITE_APP_VERSION env var.
CI sets it to branch@sha. Falls back to 'dev' in local dev.
Displayed next to "Cameleer" in the sidebar header.

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

ui/src/vite-env.d.ts

@@ -1 +1,3 @@
 /// <reference types="vite/client" />
+
+declare const __APP_VERSION__: string;