v0.1.19 - Throughput overview & responsive header

- Throughput: new "Throughput Overview" header with Gauge icon and brand-green
  badge icons on each card (Today, This week, 4-week average, Horse Mix, Grain Mix)
- Throughput: inline rolling-range selector (7d / 4w / 6w / 12w, default 4 weeks)
  driving the customer-mix cards; stats window widened to 12 weeks so switching
  range is a pure client-side re-filter
- Throughput: cards collapse to a single even 5-across row on laptop and up,
  with container-query value text that scales to each card's width
- Throughput: date logic pinned to Australian Eastern time (fixes the day-early
  date); This week subtitle shows the Mon-Sun date range
- Throughput: subtler tinted add-form; removed the inline-entry kicker and the
  "Open full form" link
- Topbar: fix cramped laptop header - action toggles no longer wrap above the
  user button; search drops to its own row earlier

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/models/ordering.py

@@ -376,6 +376,34 @@ class NotificationSetting(Base):
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow)
 
 
+class XeroContactLink(Base):
+    """Persistent link between a customer (:class:`ClientAccount`) and a Xero
+    contact.
+
+    Maintained from the Integrations console. Once a customer is linked, order
+    invoices reference the real Xero ``ContactID`` instead of falling back to the
+    client code — which is what lets Xero attach the invoice to the right
+    contact rather than creating a duplicate. One link per customer per tenant.
+    """
+
+    __tablename__ = "xero_contact_links"
+    __table_args__ = (
+        UniqueConstraint("tenant_id", "client_account_id", name="uq_xero_contact_link_customer"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    tenant_id: Mapped[str] = mapped_column(String(64), default="default", index=True)
+    client_account_id: Mapped[int] = mapped_column(ForeignKey("client_accounts.id"), index=True)
+    xero_contact_id: Mapped[str] = mapped_column(String(128))
+    xero_contact_name: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    xero_contact_email: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    # When the contact details were last reconciled with Xero (a future live
+    # sync can refresh name/email and stamp this).
+    last_synced_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow)
+    updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+
 class XeroSyncLog(Base):
     __tablename__ = "xero_sync_log"