feat(showcase): enable the record History tab on Account

The v16 sweep (#3358 §4 "Record History tab") couldn't exercise the tab because
no showcase object opted into it: the console gates the History tab on
object-level `enable.trackHistory` (RecordDetailView) — audit *capture* is
always on, but the *tab* is opt-in — and the showcase only set `trackHistory`
at the FIELD level (industry/status), which just selects which diffs are
summarized. So the tab never rendered anywhere and #2691/#3293 (display-value
diffs, computed-field exclusion) were undemonstrated.

Set `enable: { trackHistory: true }` on Account (which already declares
field-level `trackHistory` on `industry`/`status` for clean diffs).

Verified in the running app: the Account detail now shows a **历史 / History**
tab; after editing `industry`, it renders "Dev Admin · UPDATE · 行业: Retail →
Technology" — field label + select **display values** (not raw stored codes/ids),
no phantom value→null rows. `os validate` + `tsc --noEmit` pass.

Follow-up to #3364 / #3393, from the #3358 sweep §4.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: os-zhuang

examples/app-showcase/src/data/objects/account.object.ts

@@ -40,6 +40,14 @@ export const Account = ObjectSchema.create({
   // every surface, no per-page config.
   highlightFields: ['status', 'industry', 'annual_revenue'],
 
+  // Show the record **History tab** (audit-trail UI). The console gates the tab
+  // on object-level `enable.trackHistory` (RecordDetailView) — audit *capture*
+  // is always on, but the tab is opt-in — so without this the tab never renders
+  // and the feature is undemonstrable. Paired with the per-field `trackHistory`
+  // on `industry` / `status` below so those changes get summarized diffs
+  // (display values, not raw ids; computed fields excluded — #2691/#3293).
+  enable: { trackHistory: true },
+
   fields: {
     name: Field.text({ label: 'Account Name', required: true, searchable: true, maxLength: 200 }),
     industry: Field.select({