Make Keychain and shared-container state correct before the Mac port

Two failures here are silent rather than loud, which is why they have gone
unnoticed on iOS and would have been much harder to diagnose on a Mac.

The old `enum Keychain` discarded all four OSStatus results. On Mac Catalyst a
missing `keychain-access-groups` entitlement makes SecItem calls fail with
errSecMissingEntitlement, and because nothing checked, that was indistinguishable
from "no token stored" — the user would be signed out on every launch, with no
error anywhere. KeychainStore now checks every status, distinguishes
errSecItemNotFound from real failures, and sets kSecUseDataProtectionKeychain so
macOS selects the modern entitlement-gated keychain instead of the legacy login
keychain.

Adding the entitlement moves the default access group to the first array entry,
so that entry is deliberately the app's own group: existing tokens keep
resolving with an unqualified query and nobody is signed out. loadToken() then
migrates forward in three steps — shared group, own default group, and the
pre-Keychain UserDefaults copy.

If the entitlement is not provisioned yet, KeychainStore falls back to the
default group rather than throwing. A hard failure would make the app unusable
for everyone whose provisioning lags; the fallback asserts in DEBUG instead, so
a misconfiguration is loud in development and survivable in production.

Separately, logout() left widget-cache.json in the App Group container, so
widgets kept rendering the signed-out user's events indefinitely. That is an
existing iOS bug, and it would have leaked the same data to any other app
reading the container. WidgetStore.clear() now removes both cache files and
reloads the timelines.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Shared/WidgetData.swift

@@ -152,6 +152,17 @@ enum WidgetStore {
         return (try? JSONDecoder().decode([WidgetCalendar].self, from: data)) ?? []
     }
 
+    /// Drop the cached snapshot and calendar list. Called on logout and on
+    /// server reset: without this the files survive, and widgets — plus any
+    /// other app reading the group container — keep rendering the previous
+    /// user's events indefinitely.
+    static func clear() {
+        for url in [cacheURL, calendarsURL].compactMap({ $0 }) {
+            try? FileManager.default.removeItem(at: url)
+        }
+        WidgetTimelineNotifier.reload()
+    }
+
     /// Rewrite the existing snapshot with the latest colour / language values
     /// from UserDefaults. Used when the user tweaks an appearance setting and
     /// we want the widgets to refresh immediately, without needing a new event