Forms: Cmd-Enter submits from any control in a form

Form sets the ElyForm key context and binds secondary-enter to Submit
under it, so a focused tile, chip or button submits too. The prompt
enhancer's wash trims anything but letters and digits from its runs,
its spacing sits on its parts so nothing gaps at rest, and its tests
check that the field takes focus back.

TASKS.md

@@ -291,3 +291,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T24e | 2 | PASS | |
 | T24f | 1 | FAIL | Tab to a sortable header past a narrow table's edge left it out of view; a virtualized table still drew past its box at 280px |
 | T24f | 2 | PASS | |
+| T25a | 1 | FAIL | Cmd-Enter generated only while a text field held focus; the form set no key context of its own |