Answer "signed out" vs "never ran" in one read; make SnapshotTheme constructible

Signing out deletes the cached events — keeping them would leak the previous
user's calendar to anything reading the container — so an absent cache alone
cannot distinguish "signed out" from "Calendarr has never run on this machine".
Those need different messages, so read() now falls back to the session record
and reports .loggedOut when one exists and says so. Every consumer would
otherwise have to reimplement that two-step, and any that forgot would tell a
signed-out user to go install an app they already have.

SnapshotTheme only had the compiler's internal memberwise initializer, so no
consumer outside the package could actually build one.

Sources/CalendarrCore/Snapshot/SnapshotModels.swift

@@ -79,6 +79,12 @@ public struct SnapshotTheme: Sendable, Hashable {
     public let line: String
     public let primary: String
     public let accent: String
+
+    public init(today: String, text: String, background: String,
+                line: String, primary: String, accent: String) {
+        self.today = today; self.text = text; self.background = background
+        self.line = line; self.primary = primary; self.accent = accent
+    }
 }
 
 // MARK: - Snapshot