feat(#417): add session expiry detection to AuthProvider

Adds sessionExpiring and sessionMinutesRemaining to auth context.
Checks session expiry every 60s, warns when within 5 minutes.

Refs #417

apps/web/src/lib/auth/auth-context.tsx

@@ -1,6 +1,14 @@
 "use client";
 
-import { createContext, useContext, useState, useEffect, useCallback, type ReactNode } from "react";
+import {
+  createContext,
+  useContext,
+  useState,
+  useEffect,
+  useCallback,
+  useRef,
+  type ReactNode,
+} from "react";
 import type { AuthUser, AuthSession } from "@mosaic/shared";
 import { apiGet, apiPost } from "../api/client";
 
@@ -9,11 +17,19 @@ import { apiGet, apiPost } from "../api/client";
  */
 export type AuthErrorType = "network" | "backend" | null;
 
+/** Threshold in minutes before session expiry to start warning */
+const SESSION_EXPIRY_WARNING_MINUTES = 5;
+
+/** Interval in milliseconds to check session expiry */
+const SESSION_CHECK_INTERVAL_MS = 60_000;
+
 interface AuthContextValue {
   user: AuthUser | null;
   isLoading: boolean;
   isAuthenticated: boolean;
   authError: AuthErrorType;
+  sessionExpiring: boolean;
+  sessionMinutesRemaining: number;
   signOut: () => Promise<void>;
   refreshSession: () => Promise<void>;
 }
@@ -72,12 +88,23 @@ export function AuthProvider({ children }: { children: ReactNode }): React.JSX.E
   const [user, setUser] = useState<AuthUser | null>(null);
   const [isLoading, setIsLoading] = useState(true);
   const [authError, setAuthError] = useState<AuthErrorType>(null);
+  const [sessionExpiring, setSessionExpiring] = useState(false);
+  const [sessionMinutesRemaining, setSessionMinutesRemaining] = useState(0);
+  const expiresAtRef = useRef<Date | null>(null);
 
   const checkSession = useCallback(async () => {
     try {
       const session = await apiGet<AuthSession>("/auth/session");
       setUser(session.user);
       setAuthError(null);
+
+      // Track session expiry timestamp
+      if (session.session?.expiresAt) {
+        expiresAtRef.current = new Date(session.session.expiresAt);
+      }
+
+      // Reset expiring state on successful session check
+      setSessionExpiring(false);
     } catch (error) {
       const { isBackendDown, errorType } = isBackendError(error);
 
@@ -91,6 +118,8 @@ export function AuthProvider({ children }: { children: ReactNode }): React.JSX.E
       }
 
       setUser(null);
+      expiresAtRef.current = null;
+      setSessionExpiring(false);
     } finally {
       setIsLoading(false);
     }
@@ -103,6 +132,8 @@ export function AuthProvider({ children }: { children: ReactNode }): React.JSX.E
       console.error("Sign out error:", error);
     } finally {
       setUser(null);
+      expiresAtRef.current = null;
+      setSessionExpiring(false);
     }
   }, []);
 
@@ -114,11 +145,49 @@ export function AuthProvider({ children }: { children: ReactNode }): React.JSX.E
     void checkSession();
   }, [checkSession]);
 
+  // Periodically check whether the session is approaching expiry
+  useEffect((): (() => void) => {
+    if (!user || !expiresAtRef.current) {
+      return (): void => {
+        /* no-op cleanup */
+      };
+    }
+
+    const checkExpiry = (): void => {
+      if (!expiresAtRef.current) return;
+
+      const remainingMs = expiresAtRef.current.getTime() - Date.now();
+      const minutes = Math.ceil(remainingMs / 60_000);
+
+      if (minutes <= 0) {
+        // Session has expired
+        setUser(null);
+        setSessionExpiring(false);
+        setSessionMinutesRemaining(0);
+        expiresAtRef.current = null;
+      } else if (minutes <= SESSION_EXPIRY_WARNING_MINUTES) {
+        setSessionExpiring(true);
+        setSessionMinutesRemaining(minutes);
+      } else {
+        setSessionExpiring(false);
+        setSessionMinutesRemaining(minutes);
+      }
+    };
+
+    checkExpiry();
+    const interval = setInterval(checkExpiry, SESSION_CHECK_INTERVAL_MS);
+    return (): void => {
+      clearInterval(interval);
+    };
+  }, [user]);
+
   const value: AuthContextValue = {
     user,
     isLoading,
     isAuthenticated: user !== null,
     authError,
+    sessionExpiring,
+    sessionMinutesRemaining,
     signOut,
     refreshSession,
   };