[Feat]: enhance UI for 'Reply privately' feature

• Ref: https://github.com/nextcloud/spreed/issues/6301
• Web implementation: https://github.com/nextcloud/spreed/pull/17069

Resolving feature gap with server implementation after community contribution.

Things to consider:

• Required capability: private-reply (Talk 24.0.0)
• Required payload to send with POST /ocs/v2.php/apps/spreed/api/v1/chat/{token}:
  • replyTo: original message id
  • replyToToken : original conversation token
• New message metadata fields (for rendering):

 *     // Set only when a message in a convo is private replied on a 1-1 room. Represents the parent message id
 *     replyToMessageId?: int,
 *     // Set only when a message in a convo is private replied on a 1-1 room. Represents the parent message's group conversation token.
 *     replyToConversationToken?: string,
 *     // Set only when a message in a convo is private replied on a 1-1 room. Represents the parent message's group conversation name
 *     replyToConversationName?: string,
 *     // Set only when a message in a convo is private replied on a 1-1 room. Represents the parent message's actor display name
 *     replyToActorDisplayName?: string,

• Parent message is also cross-posted in 1:1 chat with VERB_PRIVATE_REPLY = 'private_reply', should be invisible (stored in DB,
• Ideally render replyTo message in 1:1 chat before sending/during sending (temporary)
• Scheduled messages are not supported atm

Start with the referenced server issue and web implementation PR, then trace the Android chat UI and its POST /ocs/v2.php/apps/spreed/api/v1/chat/{token} request. Check the private-reply capability, replyTo fields, and metadata needed to render the parent message in a 1:1 chat. Done means private replies work in the UI, including temporary rendering where feasible, while scheduled messages remain unsupported.