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>

Calendarr iOS/Calendarr iOS.entitlements

@@ -6,5 +6,15 @@
 	<array>
 		<string>group.com.scarriffleservices.calendarr</string>
 	</array>
+	<!-- Order matters. Without this entitlement the default access group is
+	     <AppIdentifierPrefix><bundle id>, which is where every existing token
+	     lives. Adding the entitlement makes the FIRST entry the new default, so
+	     listing our own group first keeps those tokens resolving and nobody
+	     gets signed out. The .shared group is the one the Mac app also reads. -->
+	<key>keychain-access-groups</key>
+	<array>
+		<string>$(AppIdentifierPrefix)com.scarriffleservices.calendarr.ios</string>
+		<string>$(AppIdentifierPrefix)com.scarriffleservices.calendarr.shared</string>
+	</array>
 </dict>
 </plist>