fix(web-ui): display all human-facing timestamps in Asia/Jerusalem (deterministic)

Storage stays UTC (DB TIMESTAMPTZ, API ISO-UTC) — only the display layer is
localized, and now deterministically: every timestamp renders pinned to
Asia/Jerusalem via a single Intl-based formatter, so SSR (UTC container) and
the browser agree on any runtime. No layout/visible-format change — only the tz.

- New single date formatter web-ui/src/lib/format-date.ts (G2): formatDate /
  formatDateShort / formatDateLong / formatDateTime / formatDateTimeFull /
  formatTime / formatIsoDate + Israel helpers getIsraelYear / israelDayKey /
  israelMidnightMs / israelParts + formatRelative (long/short/tight wording).
- Routed all ad-hoc toLocaleDateString/toLocaleTimeString/toLocaleString +
  hand-rolled new Date(...).get*() / toISOString().slice(0,10) timestamp call
  sites (30 files) through it. Number toLocaleString left untouched.
- Spec: added INV-UI9 to docs/spec/X6 (UTC storage, Asia/Jerusalem display).

Display-only; no layout/design/IA change → Claude Design gate N/A.
Invariants: G2 (single date formatter, no parallel ad-hoc formatting), INV-UI9.

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

web-ui/src/components/digests/digest-card.tsx

@@ -5,15 +5,7 @@ import type { ReactNode } from "react";
 import { Badge } from "@/components/ui/badge";
 import { practiceAreaLabel } from "@/components/precedents/practice-area";
 import type { Digest } from "@/lib/api/digests";
-
-function formatDate(iso: string | null) {
-  if (!iso) return "—";
-  try {
-    return new Date(iso).toLocaleDateString("he-IL");
-  } catch {
-    return iso;
-  }
-}
+import { formatDateShort as formatDate } from "@/lib/format-date";
 
 /**
  * Presentational card for a single digest ("כל יום" radar entry).