Devcontainer: enable bash-completion and set UTF-8 locale

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -13,7 +13,8 @@
     // Bind mounts are resolved by the host's daemon, so they need host paths:
     // use ${LOCAL_WORKSPACE_FOLDER} instead of paths inside this container.
     "remoteEnv": {
-        "LOCAL_WORKSPACE_FOLDER": "${localWorkspaceFolder}"
+        "LOCAL_WORKSPACE_FOLDER": "${localWorkspaceFolder}",
+        "LANG": "C.UTF-8"
     },
     "postCreateCommand": "pip install -U pip && pip install -U -r requirements.txt -r requirements-dev.txt",
     "forwardPorts": [