Chart builder: data-aware defaults, one-click hint fixes, canvas preview, fullscreen modal

docs/architecture/03-modal-system.md

@@ -11,7 +11,9 @@ authoritative architecture for adding, opening, closing, and rendering modals.
 - **At most one modal open at a time** (mandated by the product spec). Opening a
   modal closes any other; the two never overlap.
 - **Uniform dismissal**: close button, `Escape`, or backdrop click — never a
-  click inside the body.
+  click inside the body. A modal holding in-progress work can opt out of the
+  **backdrop** click (`dismissOnBackdrop: false`) so a stray click can't discard it
+  (the Chart Builder does); close button and `Escape` still dismiss.
 - **Accessible by default**: focus moves into the modal on open and returns to
   the trigger on close.
 - **Unsaved-change safety** for editing modals, with an explicit opt-out for
@@ -492,6 +494,13 @@ export function useFocusTrap<T extends HTMLElement = HTMLDivElement>(
 - Don't dismiss on clicks inside the body, and don't let Escape fire when no
   modal is open (the handler only exists while a modal renders).
 
+**Sizing & backdrop opt-out.** The shell picks a **size tier** by modal: a small form
+(Extract), a large two-pane manager (Datasets), or a near-fullscreen **work surface**
+(Chart Builder — a config pane plus a chart that wants room). The two larger tiers have a
+definite height so their inner panes scroll **internally** rather than the modal growing
+past the viewport. A modal opts a backdrop click out of dismissal with the registry's
+`dismissOnBackdrop: false` (above).
+
 ---
 
 ## Confirmation & alert dialogs