Fix shutdown + footer image; add interactive setup wizard + linked-identity chat

Bug fixes:
- YamlStorage: scheduleFlush now checks the plugin's enabled state
  via a BooleanSupplier and falls back to a synchronous save when
  the plugin is already disabled. This fixes the
  IllegalPluginAccessException that fired during
  TicketModule.shutdown() (which is called after Bukkit has set
  the plugin to disabled) when setData tried to schedule an async
  task.
- JoinQuitListener: footer icon is no longer hardcoded to a pair
  of imgur URLs that often fail to load. The footer icon is now
  resolved in three layers: events.<type>.footer-icon,
  events.footer-icon, then the guild icon, then null.

Experience enhancements (join/quit embeds):
- Clickable author URL (links to NameMC profile of the player).
- Player count delta in the Online field (e.g. '12/50 (was 11)').
- Session duration on quit (e.g. '1h 23m').
- First-join indicator: a 'New Player' field and a private
  welcome message (events.join.first-join-message) when
  hasPlayedBefore() returns false. Configurable via
  events.join.show-first-join-indicator.

Interactive /setup wizard:
- Rewritten SetupCommand with a 3-step ticket flow that uses
  Discord modals to add, edit, remove, and reorder ticket
  categories from inside the wizard. Operators no longer need
  to edit config.yml for the most-tweaked ticket section.
- New buttons: Add, Edit, Remove, Move up, Move down, Done.
- The category list is rebuilt with every change; ordering is
  preserved by re-writing the tickets.categories section in the
  new order.
- Done & post panel validates that at least one category exists
  before calling TicketModule.postPanel.

Linked-identity chat:
- ChatBridge now consults LinkModule and uses the player's
  Discord name + avatar (via JDA's getMemberById) when sending
  webhooks, instead of their Minecraft skin/username. Operators
  can opt out per-field with chat.prefer-linked-avatar /
  chat.prefer-linked-name (both default true). Unlinked players
  always fall back to the Crafatar avatar and MC name.
- For default nameFormat, the resolved name is
  'DiscordName (MCName)' so the channel can still tell who it is
  at a glance. A new %discord_name% placeholder is available in
  custom nameFormat strings.

Config additions:
- events.footer-icon, events.join.footer-icon, events.quit.footer-icon
- events.join.first-join-message, events.join.show-first-join-indicator
- chat.prefer-linked-avatar, chat.prefer-linked-name
- tickets.* (unchanged; the wizard writes to it)

Author: ZDiscord Maintainer