Share a single CalendarStore for unified permission state

Introduce CalendarStore.shared so Today, Calendar, and Onboarding read
the same auth status and event cache instead of each spinning up its own
EKEventStore. Trust the system's authoritative authorization status over
the granted bool, debounce repeat permission requests, route denied users
to Settings, and prompt when enabling local calendars without access.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCal.xcodeproj/xcshareddata/xcschemes/KisaniCal.xcscheme

@@ -1,11 +1,10 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1500"
-   version = "1.7">
+   LastUpgradeVersion = "2620"
+   version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"
-      buildImplicitDependencies = "YES"
-      runPostActionsOnFailure = "NO">
+      buildImplicitDependencies = "YES">
       <BuildActionEntries>
          <BuildActionEntry
             buildForTesting = "YES"
@@ -27,8 +26,7 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
-      shouldUseLaunchSchemeArgsEnv = "YES"
-      onlyGenerateCoverageForSpecifiedTargets = "NO">
+      shouldUseLaunchSchemeArgsEnv = "YES">
       <MacroExpansion>
          <BuildableReference
             BuildableIdentifier = "primary"
@@ -40,8 +38,6 @@
       </MacroExpansion>
       <Testables>
       </Testables>
-      <CommandLineArguments>
-      </CommandLineArguments>
    </TestAction>
    <LaunchAction
       buildConfiguration = "Debug"
@@ -63,8 +59,6 @@
             ReferencedContainer = "container:KisaniCal.xcodeproj">
          </BuildableReference>
       </BuildableProductRunnable>
-      <CommandLineArguments>
-      </CommandLineArguments>
    </LaunchAction>
    <ProfileAction
       buildConfiguration = "Release"
@@ -82,8 +76,6 @@
             ReferencedContainer = "container:KisaniCal.xcodeproj">
          </BuildableReference>
       </BuildableProductRunnable>
-      <CommandLineArguments>
-      </CommandLineArguments>
    </ProfileAction>
    <AnalyzeAction
       buildConfiguration = "Debug">