Stage 8: UI social/account/history surfaces

Wire the deferred Stage 7 surfaces end-to-end (UI -> gateway transcode ->
backend REST -> existing domain services): friends (incl. one-time friend
codes), per-user blocks, friend-game invitations, profile editing + email
binding, the statistics screen, and the in-game history + GCG export.

Friends gain two add paths (interview decision, a deliberate plan change):
one-time 6-digit codes (friend_codes table, 12h TTL, single-use, rate-limited
redeem); and play-gated requests (shared game required) where an explicit
decline is permanent, an ignored request lapses after 30 days, and a code
bypasses a decline. Migration 00006 widens friendships_status_chk and adds
friend_codes.

Lobby notification badge is poll + push: a new generic `notify` event drives
it live; the client polls on open/focus. Language stays a single Settings
control that writes through to the durable account's preferred_language. GCG
export is finished-only (game.ErrGameActive) and shares/downloads the .gcg file.

Tests: backend unit + inttest (friend gate/decline/code, ListInvitations,
GetStats, GCG gate), gateway transcode round-trips + notify constructor, UI
vitest (codecs, win-rate, share choice) + Playwright social specs. Docs: PLAN
(Stage 8 done + refinements + TODO-5), ARCHITECTURE, FUNCTIONAL(+ru), UI_DESIGN,
TESTING, module READMEs.

gateway/internal/transcode/encode.go

@@ -43,6 +43,8 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	name := b.CreateString(p.DisplayName)
 	lang := b.CreateString(p.PreferredLanguage)
 	tz := b.CreateString(p.TimeZone)
+	awayStart := b.CreateString(p.AwayStart)
+	awayEnd := b.CreateString(p.AwayEnd)
 	fb.ProfileStart(b)
 	fb.ProfileAddUserId(b, uid)
 	fb.ProfileAddDisplayName(b, name)
@@ -52,6 +54,8 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	fb.ProfileAddBlockChat(b, p.BlockChat)
 	fb.ProfileAddBlockFriendRequests(b, p.BlockFriendRequests)
 	fb.ProfileAddIsGuest(b, p.IsGuest)
+	fb.ProfileAddAwayStart(b, awayStart)
+	fb.ProfileAddAwayEnd(b, awayEnd)
 	b.Finish(fb.ProfileEnd(b))
 	return b.FinishedBytes()
 }