Send forwarded media to the forum topic picked in the share sheet

The picker only read the selected dialog ids, so media always landed in
a forum's General topic. Read ShareAlert's selectedDialogTopics as well
and address each target the way ShareAlert.sendInternal() does: reply to
the topic's start message in forums, or set monoForumPeer in a channel's
direct-messages chat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/media-viewer-notes.md

@@ -84,6 +84,25 @@ otherwise. On Android the dialog id of a channel is `-chat.id`, with no `-100`
 prefix. DMs, basic groups and Saved Messages have no link, so media forwarded
 from rich messages there is sent without a caption.
 
+## Forum topics in the share sheet
+
+When a forum is picked in `ShareAlert`, the topic lands in the protected
+`selectedDialogTopics` map. That map is keyed by the `TLRPC.Dialog` values of
+`selectedDialogs`, not by dialog id. `ShareAlert.sendInternal()` then addresses
+each chat in one of two ways:
+
+- **Forum topic:** wraps `topic.topicStartMessage` in a `MessageObject`, sets
+  `isTopicMainMessage = true`, and passes it as both `replyToMsg` and
+  `replyToTopMsg`. `SendMessagesHelper` sends its id as `top_msg_id`. A topic's
+  id is its start message's id.
+- **Mono forum** (a channel's direct-messages chat,
+  `MessagesController.isMonoForum`): no reply message. Instead
+  `monoForumPeer = DialogObject.getPeerDialogId(topic.from_id)`.
+
+A plugin that only reads the `selectedDialogs` keys sends to the chat's
+General topic. `picker._selected_targets` reproduces the logic above inside
+`didShare()`, which runs before the sheet clears its selection.
+
 ## File references
 
 Pass the adapter's parent object (WebPage or MessageObject) as `parentObject`