fix(v0.5.0): pre-release polish from full-codebase review

- Plugin.cs: mark RetentionSweepRunning volatile so the ImGui thread
  reads the latest value without a stale register-cached copy
- EmoteCache.cs: reset State to Unloaded on exception so a later
  trigger can retry instead of being blocked by the early-out
- Settings.cs: switch the SaveAndClose / Discard buttons to Allman
  bracing for consistency with the rest of the file, and include the
  ItemSpacing in the Ko-fi-button right-edge calculation
- Privacy.cs: add a saved-policy hint above the manual retention
  Ctrl+Shift button so the existing Cleanup wording pattern is
  matched here too
- HellionStrings: drop seven unreferenced keys (Theme_Heading,
  Migration_Notification_*, Migration_Webinterface_Removed_*,
  AutoTellTabs_Migration_*) and their EN/DE values, add the new
  Retention_Help_SavedNote string

ChatTwo/Ui/SettingsTabs/Privacy.cs

@@ -403,6 +403,9 @@ internal sealed class Privacy : ISettingsTab
 
                 ImGui.Spacing();
 
+                ImGuiUtil.HelpText(HellionStrings.Retention_Help_SavedNote);
+                ImGui.Spacing();
+
                 using (ImRaii.Disabled(RetentionRunning))
                 {
                     if (ImGuiUtil.CtrlShiftButton(HellionStrings.Retention_Apply_Label, HellionStrings.Retention_Apply_Tooltip))