refactor(client-ui-plugin-config): stage card edits behind an explicit save

Controls committed on blur, which turned leaving a field into a durable,
revision-fenced document write the user could neither preview nor undo, and
silently discarded a draft the field did not accept.

A card's form now owns the staged text every control renders, and Save is the
only point where drafts become writes. Reset stages the composed default the
same way; an invalid draft blocks the save with its reason instead of being
dropped; Discard drops the drafts; a collapsed card marks that it holds some.
The Host stays the only authority on whether a value was accepted, so the save
reads the section back and keeps the drafts of a save that did not land.

packages/client/ui-plugin-config/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-plugin-config/README.md
-README.md: e830589b5e279fb0bdda23591502989bebc2a336
-README.zh.md: e614d7b6858f8cbf2a38cb7397b5d8f93445ab6c
+README.md: 51d86e8fbfd19d3d37f68650163180751fb27061
+README.zh.md: 48a68900a20aaaabcc9763b5aa61bc23cf3d16d8