feat(apps): reclaim @objectstack/account for console Account app; drop dead SPA (ADR-0048)

Removes the deprecated standalone account-portal SPA (apps/account) and reclaims
the @objectstack/account name for the console Account app as its own package
(`com.objectstack.account`, namespace `account`). Boot-neutral skeleton (mirrors
studio/setup): thin AccountAppPlugin, transitional import from platform-objects,
not yet wired into dev/serve. CLI's vestigial workspace dep now resolves to the
reclaimed package (no code imported the old SPA). Builds clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: os-zhuang

.changeset/adr-0048-split-account-app.md

@@ -0,0 +1,12 @@
+---
+"@objectstack/account": minor
+---
+
+feat(apps): reclaim `@objectstack/account` for the console Account app (ADR-0048)
+
+Removes the deprecated standalone account-portal SPA (`apps/account`) and
+reclaims the `@objectstack/account` name for the console Account app as its own
+ObjectStack package (`com.objectstack.account`, namespace `account`) per
+ADR-0048 "one app per package". Boot-neutral skeleton (transitional import from
+platform-objects; not yet wired into the dev/serve plugin set — that switch
+lands in a follow-up verified against a live `os dev` boot).