feat: add floating call widget with SIP controls, click-to-call, and SIP provider

Introduce a shared SipProvider context so all components use the same SIP
connection. Add a floating CallWidget (idle pill, ringing, active with
disposition, ended states) visible for CC agents on every page. Add a
ClickToCallButton for the worklist. Wire SIP status badge and worklist
into the call-desk page.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/call-desk/click-to-call-button.tsx

@@ -0,0 +1,25 @@
+import { Phone01 } from '@untitledui/icons';
+import { Button } from '@/components/base/buttons/button';
+import { useSip } from '@/providers/sip-provider';
+
+interface ClickToCallButtonProps {
+    phoneNumber: string;
+    label?: string;
+    size?: 'sm' | 'md';
+}
+
+export const ClickToCallButton = ({ phoneNumber, label, size = 'sm' }: ClickToCallButtonProps) => {
+    const { makeCall, isRegistered, isInCall } = useSip();
+
+    return (
+        <Button
+            size={size}
+            color="primary"
+            iconLeading={Phone01}
+            onClick={() => makeCall(phoneNumber)}
+            isDisabled={!isRegistered || isInCall || phoneNumber === ''}
+        >
+            {label ?? 'Call'}
+        </Button>
+    );
+};