Add message export for GDPR Art. 15 right of access

The privacy story is incomplete without a way to actually hand the
data over. New Export section in the Privacy tab streams matching
messages to a Markdown, JSON or CSV file using Dalamud's file
dialog and a background thread, so the settings UI stays
responsive even when the export crawls a 150k-message archive.

MessageStore.StreamForExport returns a MessageEnumerator over
non-deleted rows filtered by ChatType list and date range, sorted
ascending. MessageExporter.ExportToFile takes that enumerator,
optionally narrows by SenderSource.TextValue substring (case-
insensitive), and writes one of three formats:

  Markdown — human-readable, day headers, [HH:mm] ChatType Sender:
  prefix per line, trailing total.

  JSON — single object with metadata (filter snapshot, exported_at,
  plugin name) and a messages array carrying id, ISO-8601 date,
  numeric and named ChatType, source/target kinds, receiver,
  content_id, sender plaintext, content plaintext.

  CSV — header line plus quoted-when-needed rows for spreadsheet
  ingestion.

Sender plaintext, channel filter, date range and format are
exposed as form fields above the Export button. Empty channel
selection means "all stored channels", a 0-day range means "no
time limit". Result count and target path are reported via
WrapperUtil notifications.

ChatTwo/MessageStore.cs

@@ -500,6 +500,54 @@ internal class MessageStore : IDisposable
         cmd.ExecuteNonQuery();
     }
 
+    /// <summary>
+    /// Streams messages for export. Optional filters:
+    /// - <paramref name="chatTypes"/>: limit to these ChatTypes
+    /// - <paramref name="from"/> / <paramref name="to"/>: inclusive date range
+    /// Result is sorted ascending by Date and excludes soft-deleted rows.
+    /// Caller is responsible for disposing the enumerator.
+    /// </summary>
+    internal MessageEnumerator StreamForExport(
+        IReadOnlyCollection<int>? chatTypes,
+        DateTimeOffset? from,
+        DateTimeOffset? to)
+    {
+        var clauses = new List<string> { "deleted = false" };
+        if (chatTypes is { Count: > 0 })
+            clauses.Add($"ChatType IN ({string.Join(",", chatTypes)})");
+        if (from is not null)
+            clauses.Add("Date >= $From");
+        if (to is not null)
+            clauses.Add("Date <= $To");
+
+        var cmd = Connection.CreateCommand();
+        cmd.CommandText = @"
+            SELECT
+                Id,
+                Receiver,
+                ContentId,
+                Date,
+                ChatType,
+                SourceKind,
+                TargetKind,
+                Sender,
+                Content,
+                SenderSource,
+                ContentSource,
+                ExtraChatChannel
+            FROM messages
+            WHERE " + string.Join(" AND ", clauses) + @"
+            ORDER BY Date ASC;";
+        cmd.CommandTimeout = 600;
+
+        if (from is not null)
+            cmd.Parameters.AddWithValue("$From", from.Value.ToUnixTimeMilliseconds());
+        if (to is not null)
+            cmd.Parameters.AddWithValue("$To", to.Value.ToUnixTimeMilliseconds());
+
+        return new MessageEnumerator(cmd.ExecuteReader());
+    }
+
     /// <summary>
     /// Get the most recent messages.
     /// </summary>