fix: timezone handling across fullstack — UTC parsing + display in configured TZ

Root cause: backend returned naive ISO datetimes (no Z/offset) → JS parsed them as
browser local time → 3h drift for MSK users → '3ч' instead of 'только что'.

Backend:
- schemas/agent.py: add field_serializer('Z' suffix) for ConversationItem.updated_at
  and MessageItem.created_at — naive datetimes serialized as UTC
- routes/agent_conversations.py: datetime.utcnow() → datetime.now(timezone.utc) (3x)

Frontend:
- New: stores/timezone.svelte.ts — global reactive appTimezone store
- dateFormat.ts: add parseDateUTC() (appends 'Z' to naive ISO), all format*()
  functions now use parseDateUTC + { timeZone: appTimezone.current }
- ~25 files: replace new Date(apiString) → parseDateUTC(apiString),
  add timeZone: appTimezone.current to toLocaleString()/toLocaleDateString()
- SystemSettings.svelte + HealthCenterModel sync appTimezone to global store
- ConversationList.svelte: fix relativeTime() and date grouping (the '3ч' bug)

Verified: backend schema test, frontend 2501 tests pass, build succeeds,
browser validation on /agent and /settings.

backend/src/api/routes/agent_conversations.py

@@ -2,7 +2,7 @@
 # #region AgentChat.Api.Conversations [C:3] [TYPE Module] [SEMANTICS agent-chat,api,rest]
 # @defgroup AgentChat REST routes for conversation lifecycle.
 
-from datetime import datetime
+from datetime import datetime, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy.orm import Session
@@ -170,10 +170,10 @@ async def save_conversation(
             id=body.conversation_id,
             user_id=user_id,
             title=body.title or "",
-            created_at=datetime.utcnow(),
+            created_at=datetime.now(timezone.utc),
         )
         db.add(conv)
-    conv.updated_at = datetime.utcnow()
+    conv.updated_at = datetime.now(timezone.utc)
     if body.title:
         conv.title = body.title
 
@@ -196,7 +196,7 @@ async def save_conversation(
                     state=msg_data.get("state"),
                     tool_calls=msg_data.get("tool_calls"),
                     attachments=msg_data.get("attachments"),
-                    created_at=datetime.utcnow(),
+                    created_at=datetime.now(timezone.utc),
                 )
                 db.add(msg)
         db.flush()