feat(render): card owner pill + short ISO date span

closes board-viewer-card-meta-fix

Review found two acceptance fields missing from cards:
- project_owner was in JSON but never displayed
- last_commit_iso was only consumed for relative age; no absolute date

Now both rendered (server-side src/render.ts + client-side static/board.js
in sync), with 4 new TDD tests:
- renders owner pill when project_owner is set
- omits owner pill when empty
- renders short YYYY-MM-DD date when last_commit_iso is set
- omits date span when null

CSS: .badge.owner = transparent + border (visually distinct from solid
.badge.project), .date = small muted tabular-nums. Smoke-verified in browser.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

static/board.css

@@ -245,3 +245,15 @@ aside.drawer[hidden] {
 .badge.status {
   background: var(--badge-bg);
 }
+
+.badge.owner {
+  background: transparent;
+  border: 1px solid var(--border);
+  color: var(--muted);
+}
+
+.date {
+  font-variant-numeric: tabular-nums;
+  color: var(--muted);
+  font-size: 0.7rem;
+}