Summary

• Expanded Mongo document cards list top-level fields instead of a read-only JSON blob.
• Tapping a non-_id field opens the shared Cell Inspector with Save to DB, which $sets that field and reloads the card list.
• Inspector Apply is unchanged when Save to DB is not wired (SQL grid).
• Shared encode/decode for NULL / bool / number / JSON so nested objects stay valid in the inspector.

Closes #763

Test plan

• Expand a Mongo document card, tap a scalar field, Save to DB, confirm the card reloads with the new value.
• Nested object/array field: inspector shows pretty JSON; Save to DB stores a map/list, not a string.
• _id is not tappable / not saved.
• SQL Cell Inspector (no Save to DB callback) still has a single Apply button.
• flutter test test/features/mongodb/mongo_field_codec_test.dart test/features/workspace/grid_cell_editor_test.dart