fix(jupyter): explicitly unset JUPYTERLAB_DIR to prevent inherited override

Three places in start.sh carried comments stating JUPYTERLAB_DIR 'must
stay unset' / 'intentionally unset' / 'do NOT override', but none of
them actually called unset. A value inherited from a parent environment
(old bashrc, exported env var, or runtime injection) would therefore
survive and override JupyterLab's prebuilt application assets dir,
breaking the UI with 'application assets not found' (the regression
fixed in 467fd00, which only removed the export).

Add an explicit 'unset JUPYTERLAB_DIR' at each of the three sites:
- top-level env block (line ~648)
- start_jupyter_once() env block (line ~1829)
- the third env block (line ~1925)

unset on an already-unset var is a no-op, so this is safe.

start.sh

@@ -643,6 +643,9 @@ export JUPYTER_RUNTIME_DIR="${JUPYTER_RUNTIME_DIR:-$HC_WRITABLE_BASE/.local/shar
 # NOTE: JUPYTERLAB_DIR must stay unset — it is JupyterLab's prebuilt application
 # assets dir (shipped with the pip package), not a settings dir. Overriding it
 # (as d085e58 did) breaks the UI with "application assets not found".
+# Explicitly unset so an inherited value (old bashrc, runtime injection) can't
+# break asset loading.
+unset JUPYTERLAB_DIR
 export JUPYTERLAB_SETTINGS_DIR="${JUPYTERLAB_SETTINGS_DIR:-$JUPYTER_CONFIG_DIR/lab/user-settings}"
 export JUPYTERLAB_WORKSPACES_DIR="${JUPYTERLAB_WORKSPACES_DIR:-$JUPYTER_CONFIG_DIR/lab/workspaces}"
 export JUPYTER_PREFER_ENV_PATH="${JUPYTER_PREFER_ENV_PATH:-0}"
@@ -1823,6 +1826,7 @@ start_jupyter_once() {
   export JUPYTER_DATA_DIR="${JUPYTER_DATA_DIR:-$HC_WRITABLE_BASE/.local/share/jupyter}"
   export JUPYTER_RUNTIME_DIR="${JUPYTER_RUNTIME_DIR:-$HC_WRITABLE_BASE/.local/share/jupyter/runtime}"
   # JUPYTERLAB_DIR intentionally unset — see top-level env block.
+  unset JUPYTERLAB_DIR
   export JUPYTERLAB_SETTINGS_DIR="${JUPYTERLAB_SETTINGS_DIR:-$JUPYTER_CONFIG_DIR/lab/user-settings}"
   export JUPYTERLAB_WORKSPACES_DIR="${JUPYTERLAB_WORKSPACES_DIR:-$JUPYTER_CONFIG_DIR/lab/workspaces}"
   export JUPYTER_PREFER_ENV_PATH="${JUPYTER_PREFER_ENV_PATH:-0}"
@@ -1918,6 +1922,7 @@ export JUPYTER_CONFIG_DIR="${JUPYTER_CONFIG_DIR:-$HC_WRITABLE_BASE/.jupyter}"
 export JUPYTER_DATA_DIR="${JUPYTER_DATA_DIR:-$HC_WRITABLE_BASE/.local/share/jupyter}"
 export JUPYTER_RUNTIME_DIR="${JUPYTER_RUNTIME_DIR:-$HC_WRITABLE_BASE/.local/share/jupyter/runtime}"
 # Do NOT override JUPYTERLAB_DIR — see top-level env block for the reason.
+unset JUPYTERLAB_DIR
 export JUPYTERLAB_SETTINGS_DIR="${JUPYTERLAB_SETTINGS_DIR:-$JUPYTER_CONFIG_DIR/lab/user-settings}"
 export JUPYTERLAB_WORKSPACES_DIR="${JUPYTERLAB_WORKSPACES_DIR:-$JUPYTER_CONFIG_DIR/lab/workspaces}"
 export JUPYTER_PREFER_ENV_PATH="${JUPYTER_PREFER_ENV_PATH:-0}"