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/cases/agent-activity-feed.tsx

@@ -17,6 +17,7 @@ import type {
   InteractionTask,
   PaperclipComment,
 } from "@/lib/api/agents";
+import { formatRelative } from "@/lib/format-date";
 import { toast } from "sonner";
 import {
   Bot,
@@ -83,15 +84,7 @@ function issueStatusLabel(status: string) {
 /* ── Time formatting ─────────────────────────────────────────── */
 
 function timeAgo(iso: string | null): string {
-  if (!iso) return "";
-  const diff = Date.now() - new Date(iso).getTime();
-  const mins = Math.floor(diff / 60_000);
-  if (mins < 1) return "עכשיו";
-  if (mins < 60) return `לפני ${mins} דק׳`;
-  const hours = Math.floor(mins / 60);
-  if (hours < 24) return `לפני ${hours} שע׳`;
-  const days = Math.floor(hours / 24);
-  return `לפני ${days} ימים`;
+  return formatRelative(iso, { units: "short", fallback: "" });
 }
 
 /* ── Issue identifier → find matching identifier ─────────────── */