v2.3.3: fix Discord webhook always failing (Cloudflare blocks Python UA)

Cloudflare protects Discord webhook endpoints and rejects requests
with the default Python urllib User-Agent ('Python-urllib/3.x') with
HTTP 403 + 'error code 1010'. Add a browser-like User-Agent header
to the request.

The onboarding wizard's 'Send Test Message' button (and clock-in/out
push messages) now succeed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/version.py

@@ -4,4 +4,4 @@
 릴리스 시 이 값을 올린 후 git tag → push.
 CHANGELOG.md의 최상단 항목과 일치시킬 것.
 """
-__version__ = '2.3.2'
+__version__ = '2.3.3'