Added mailboxes as local data sources (#1022)

app/MindWork AI Studio/Settings/DataModel/MailboxMaxAge.cs

@@ -0,0 +1,19 @@
+namespace AIStudio.Settings.DataModel;
+
+/// <summary>
+/// How far back the index of a mailbox reaches, by the date the server received a mail. Flagged mails are indexed regardless of their age.
+/// </summary>
+public enum MailboxMaxAge
+{
+    // The shortest period is deliberately the member with the underlying value 0: when the settings
+    // file holds a value TolerantEnumConverter cannot read, it falls back to that member. A shorter
+    // period than the user chose removes mails from the index, which the protection against mass
+    // removal asks about first. A longer one would embed mails nobody asked for, which takes hours
+    // for a large mailbox and costs money with a cloud embedding.
+    LAST_3_MONTHS = 0,
+
+    LAST_6_MONTHS,
+    LAST_12_MONTHS,
+    LAST_24_MONTHS,
+    ALL,
+}