Add first-run wizard with three privacy profiles

Fresh installs now open a setup window on first plugin load that
asks the user to pick one of three starting profiles. Existing
ChatTwo users keep skipping the wizard because the v6→v7 migration
sets Configuration.FirstRunCompleted = true on the same pass that
seeds the Privacy-First defaults — they already saw the migration
notification and can reopen the wizard from the Privacy tab if
they want to choose differently.

The three profiles map to concrete configuration sets:

  Privacy-First (recommended): own-conversation whitelist (30
  channels), retention enabled with the spec defaults (Tells 365
  days, own-conversation channels 90, fallback 30).

  Casual: Privacy-First plus public chat (Say/Shout/Yell, both
  emote types, Novice Network) with a 1-day retention window so
  RP players can scroll back the last scene without keeping
  third-party speech forever.

  Full History: filter off, retention off, GDPR warning shown
  inline. Behaves like upstream Chat 2.

The wizard window is non-modal but covers a wide layout (three
side-by-side cards) and closing it without picking anything is
treated as accepting whatever defaults are already in place. The
Privacy tab gains a "show wizard again" button at the top so the
choice is reversible.

ChatTwo/Ui/SettingsTabs/Privacy.cs

@@ -58,6 +58,10 @@ internal sealed class Privacy : ISettingsTab
 
     public void Draw(bool changed)
     {
+        if (ImGui.Button(HellionStrings.Wizard_Reopen_Button))
+            Plugin.FirstRunWizard.IsOpen = true;
+        ImGui.Spacing();
+
         ImGuiUtil.OptionCheckbox(
             ref Mutable.PrivacyFilterEnabled,
             HellionStrings.Privacy_FilterEnabled_Name,