feat(client): add STATION_CODE env var and config attribute

Reads STATION_CODE from the environment and exposes it as Config.STATION_CODE
(None when unset or empty). Adds the variable to .env.example with a
per-station deployment note, and covers both read and missing-key paths with
new pytest tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -20,6 +20,10 @@ CLIENT_HOST=0.0.0.0
 CLIENT_PORT=5000
 CLIENT_SECRET_KEY=change-this-to-another-random-secret-key
 API_SERVER_URL=http://localhost:8000
+# Station code this client container belongs to (e.g. ST-001).
+# Each physical tablet/PC deployment must set this unique per-station value.
+# Leave empty only for a single-station all-in-one demo using ST-DEFAULT.
+STATION_CODE=ST-DEFAULT
 
 # --- File Storage ---
 UPLOAD_DIR=server/uploads