fix(auth): reject expired JWT on session read (silent-401 shell)

readFromStorage validated token shape but never checked exp, so an expired
token mounted the full authed shell and every API call 401d silently. Decode
the JWT and treat an expired token as no session. Pattern backported from
skyai-finance. Frontend audit 2026-06-20, rank 1.

Also clears the localStorage Session in onUnauthorized (root.tsx) so a 401
fully logs out instead of leaving a dead session behind getToken.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/lib/session.ts

@@ -44,6 +44,10 @@ function readFromStorage(): Session | null {
       typeof parsed.token !== "string"
     )
       return null
+    // An expired JWT is not a session: without this the shell renders as
+    // "logged in" and every API call 401s silently. Treat it as null so the
+    // app bounces to /login. (Frontend audit 2026-06-20.)
+    if (isTokenExpired(parsed.token)) return null
     return {
       userId: parsed.userId,
       name:
@@ -76,6 +80,35 @@ export function loadSession(): Session | null {
   return readFromStorage()
 }
 
+// A token counts as expired only if it's a JWT carrying an `exp` in the past
+// (minus a small clock-skew grace). Non-JWT dev/mock tokens (no decodable
+// `exp`) are treated as non-expiring so offline/test flows keep working.
+const TOKEN_EXPIRY_SKEW_S = 30
+export function isTokenExpired(token: string | undefined | null): boolean {
+  if (!token) return true
+  const claims = decodeJwtPayload(token)
+  const exp =
+    claims && typeof claims.exp === "number" ? (claims.exp as number) : null
+  if (exp === null) return false
+  return Date.now() / 1000 >= exp - TOKEN_EXPIRY_SKEW_S
+}
+
+function decodeJwtPayload(token: string): Record<string, unknown> | null {
+  const parts = token.split(".")
+  if (parts.length !== 3) return null
+  try {
+    const b64 = parts[1].replace(/-/g, "+").replace(/_/g, "/")
+    const pad = b64.length % 4 === 0 ? "" : "=".repeat(4 - (b64.length % 4))
+    const json =
+      typeof atob === "function"
+        ? atob(b64 + pad)
+        : Buffer.from(b64 + pad, "base64").toString("utf-8")
+    return JSON.parse(json) as Record<string, unknown>
+  } catch {
+    return null
+  }
+}
+
 export function signOut() {
   if (typeof window === "undefined") return
   localStorage.removeItem(STORAGE_KEY)