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/training/chat-panel.tsx

@@ -41,6 +41,7 @@ import {
   useDeleteChat,
   type ChatMessage,
 } from "@/lib/api/training";
+import { formatDateShort } from "@/lib/format-date";
 import { useQueryClient } from "@tanstack/react-query";
 
 export function ChatPanel() {
@@ -200,7 +201,7 @@ function ConversationsSidebar({
                       <span className="tabular-nums">{c.message_count}</span>
                       <MessageSquare className="w-3 h-3" />
                       <span className="grow text-end">
-                        {new Date(c.last_message_at).toLocaleDateString("he-IL")}
+                        {formatDateShort(c.last_message_at)}
                       </span>
                       <button
                         onClick={(e) => { e.stopPropagation(); onDelete(c.id); }}