harden: widget edge cases — expired session, account switch, cache

1. Session expires: widget gets 401 → clears stale cookie from
   App Group → stops retrying with bad auth → shows cached data
   until user opens app and re-authenticates

2. Account switch: login() now calls clearWidgetAuth() BEFORE
   syncCookieToWidget() — clears previous user's cached calories
   before writing new user's cookie. No brief display of wrong data.

3. Logout: already correct — clearWidgetAuth removes cookie +
   cached data, widget shows 0/2000

4. Minimum data: only session cookie + 2 cached numbers + timestamp
   in App Group. No passwords, no user IDs, no PII.

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

ios/Platform/PlatformWidget/PlatformWidget.swift

@@ -116,8 +116,15 @@ struct CalorieProvider: TimelineProvider {
 
         do {
             let (data, response) = try await URLSession.shared.data(for: request)
-            guard let http = response as? HTTPURLResponse,
-                  (200...299).contains(http.statusCode) else { return nil }
+            guard let http = response as? HTTPURLResponse else { return nil }
+
+            // Session expired — clear stale cookie so we don't retry
+            if http.statusCode == 401 {
+                sharedDefaults.removeObject(forKey: "widget_sessionCookie")
+                return nil
+            }
+
+            guard (200...299).contains(http.statusCode) else { return nil }
             return try JSONSerialization.jsonObject(with: data) as? [String: Any]
         } catch {
             return nil