fix: fix timezone handling across backend and frontend (v0.3.1)

- Add tz.py with local_strftime/to_local helpers that read system.timezone
  from DB and convert UTC datetimes to venue local time before formatting
- Fix all strftime() calls in orders.py, reports.py, printer_service.py
  that were formatting UTC datetimes without timezone conversion
- Fix get_order endpoint returning raw dicts without Z suffix on datetimes,
  causing JS new Date() to treat timestamps as local instead of UTC
- Fix fmtDate() in tokens.js that stripped the T separator before parsing,
  breaking UTC-to-local conversion for all report date displays
- Make open/partially_paid table chips more visually distinct on dashboard

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/routers/orders.py

@@ -1,6 +1,16 @@
 import json
 from datetime import datetime, timezone
 from fastapi import APIRouter, Depends, HTTPException, status, BackgroundTasks
+from tz import local_strftime
+
+
+def _iso(dt) -> str | None:
+    """Serialize a datetime to ISO 8601 with Z suffix so browsers parse it as UTC."""
+    if dt is None:
+        return None
+    if dt.tzinfo is None:
+        return dt.isoformat() + "Z"
+    return dt.isoformat()
 from sqlalchemy.orm import Session
 from typing import List, Optional
 
@@ -163,11 +173,11 @@ def get_order(order_id: int, db: Session = Depends(get_db), user: User = Depends
             "removed_ingredients": i.removed_ingredients,
             "notes": i.notes,
             "status": i.status,
-            "added_at": i.added_at,
+            "added_at": _iso(i.added_at),
             "printed": i.printed,
             "paid_by": i.paid_by,
             "paid_by_name": name_map.get(i.paid_by) if i.paid_by else None,
-            "paid_at": i.paid_at,
+            "paid_at": _iso(i.paid_at),
             "payment_method": i.payment_method,
             "paid_in_shift_id": i.paid_in_shift_id,
         }
@@ -183,8 +193,8 @@ def get_order(order_id: int, db: Session = Depends(get_db), user: User = Depends
             "amount": l.amount,
             "payment_method": l.payment_method,
             "note": l.note,
-            "created_at": l.created_at,
-            "offline_at": l.offline_at,
+            "created_at": _iso(l.created_at),
+            "offline_at": _iso(l.offline_at) if isinstance(l.offline_at, datetime) else l.offline_at,
             "is_duplicate": l.is_duplicate,
         }
 
@@ -205,9 +215,9 @@ def get_order(order_id: int, db: Session = Depends(get_db), user: User = Depends
         "table_id": order.table_id,
         "table_name": table_name,
         "opened_by": order.opened_by,
-        "opened_at": order.opened_at,
+        "opened_at": _iso(order.opened_at),
         "status": order.status,
-        "closed_at": order.closed_at,
+        "closed_at": _iso(order.closed_at),
         "closed_by": order.closed_by,
         "notes": order.notes,
         "business_day_id": order.business_day_id,
@@ -657,8 +667,8 @@ def print_order(
         "order_id": order.id,
         "table_name": table_name,
         "waiter_name": waiter_name,
-        "opened_at": order.opened_at.strftime("%d/%m/%Y %H:%M"),
-        "closed_at": order.closed_at.strftime("%d/%m/%Y %H:%M") if order.closed_at else None,
+        "opened_at": local_strftime(order.opened_at, "%d/%m/%Y %H:%M"),
+        "closed_at": local_strftime(order.closed_at, "%d/%m/%Y %H:%M") if order.closed_at else None,
         "status": order.status,
         "items": items_data,
         "total": grand_total,
@@ -953,7 +963,7 @@ def print_synopsis(
         "order_id": order.id,
         "table_name": table_name,
         "waiter_name": waiter_name,
-        "opened_at": order.opened_at.strftime("%d/%m/%Y %H:%M"),
+        "opened_at": local_strftime(order.opened_at, "%d/%m/%Y %H:%M"),
         "items": items_data,
         "total": total,
         "paid_total": paid_total,