Upstream replaced CliFx.Attributes with CliFx.Binding and now
requires command classes to be partial. Pipe() was also moved
from DiscordChatExporter.Core.Utils.Extensions to PowerKit.Extensions.
Introduce a message-deletion feature across CLI and GUI. Adds a new CLI command (deletemessages) with channel/before/after options and console progress. Extends DiscordClient with GetCurrentUserAsync and DeleteMessageAsync (including rate-limit handling) to perform deletions and surface authorization outcomes. GUI additions include DeleteSetup dialog, its ViewModel, view, and wiring: App registration, View/ViewModel managers, DashboardViewModel command, and a Delete button in the dashboard; deletion runs per-channel (parallel, with progress) and reports success/failure summaries.
