fix: mount memory-backed emptyDir at /dev/shm for Electron apps

Instead of disabling shared memory usage, mount a proper tmpfs at
/dev/shm so Antigravity (and Chrome) have real shared memory available.
Removes --disable-dev-shm-usage; keeps --no-sandbox (separate issue).

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

scripts/startapp.sh

@@ -21,8 +21,8 @@ echo "Workspace: $WORKSPACE_DIR"
 case "$IDE" in
     antigravity)
         echo "Opening Google Antigravity in: $WORKSPACE_DIR"
-        # --no-sandbox and --disable-dev-shm-usage are required for Electron apps in Docker
-        exec antigravity --no-sandbox --disable-dev-shm-usage --new-window --wait "$WORKSPACE_DIR"
+        # --no-sandbox is required for Electron apps in Docker (no kernel sandbox available)
+        exec antigravity --no-sandbox --new-window --wait "$WORKSPACE_DIR"
         ;;
     none)
         echo "IDE=none: no IDE launched, keeping container alive."