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/tz.py

@@ -0,0 +1,39 @@
+"""Timezone utilities for converting UTC datetimes to the venue's local time."""
+from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
+from datetime import datetime, timezone
+
+from database import SessionLocal
+from models.settings import PosSettings
+
+_FALLBACK = "Europe/Athens"
+
+
+def _get_tz_name() -> str:
+    db = SessionLocal()
+    try:
+        row = db.query(PosSettings).filter(PosSettings.key == "system.timezone").first()
+        return row.value if row and row.value else _FALLBACK
+    except Exception:
+        return _FALLBACK
+    finally:
+        db.close()
+
+
+def to_local(dt: datetime) -> datetime:
+    """Convert a UTC-aware datetime to the venue's configured local timezone."""
+    if dt is None:
+        return dt
+    if dt.tzinfo is None:
+        dt = dt.replace(tzinfo=timezone.utc)
+    try:
+        tz = ZoneInfo(_get_tz_name())
+    except ZoneInfoNotFoundError:
+        tz = ZoneInfo(_FALLBACK)
+    return dt.astimezone(tz)
+
+
+def local_strftime(dt: datetime, fmt: str) -> str:
+    """Format a UTC-aware datetime in local time using the venue's timezone."""
+    if dt is None:
+        return "—"
+    return to_local(dt).strftime(fmt)