feat(game): race exit warnings in the turn report (#12)

Surface the inactivity-removal countdown the rules promise but the
engine never reported. A race within five turns of being auto-removed
for inactivity gets a personal warning in its own report; every race
within three turns is listed publicly to all participants.

- model: Report.PersonalExitWarning + RacesLeavingSoon ([]RaceExitNotice)
- fbs: RaceExitNotice table + Report.personal_exit_warning /
  races_leaving_soon (regenerated Go + TS bindings)
- transcoder: encode/decode both fields
- engine: ReportExitWarnings fills the recipient's TTL (1..5) and lists
  other non-extinct races with TTL 1..3, excluding the recipient itself
- ui: danger-styled personal banner + "races leaving soon" section
  (hidden when empty), wired into the report view, EN/RU i18n
- docs: rules.txt report-section list, FUNCTIONAL.md 6.4 + RU mirror

Voluntary quit and idle timeout share the TTL countdown and are not
distinguished, per the agreed scope.

pkg/schema/fbs/report.fbs

@@ -209,6 +209,13 @@ table BattleSummary {
   shots:uint64;
 }
 
+// RaceExitNotice is a public notice that a race is within a few turns of being
+// auto-removed for inactivity; turns_left is the number of turns until removal.
+table RaceExitNotice {
+  race:string;
+  turns_left:uint32;
+}
+
 table Report {
   version:uint64;
   turn:uint64;
@@ -236,6 +243,8 @@ table Report {
   local_group:[LocalGroup];
   other_group:[OtherGroup];
   unidentified_group:[UnidentifiedGroup];
+  personal_exit_warning:uint32 = 0;
+  races_leaving_soon:[RaceExitNotice];
 }
 
 // GameReportRequest is the signed-gRPC request payload for