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.

ui/src/App.svelte

@@ -10,6 +10,8 @@
   import Profile from './screens/Profile.svelte';
   import Settings from './screens/Settings.svelte';
   import About from './screens/About.svelte';
+  import Friends from './screens/Friends.svelte';
+  import Stats from './screens/Stats.svelte';
   import Game from './game/Game.svelte';
 
   onMount(() => {
@@ -31,6 +33,10 @@
   <Settings />
 {:else if router.route.name === 'about'}
   <About />
+{:else if router.route.name === 'friends'}
+  <Friends />
+{:else if router.route.name === 'stats'}
+  <Stats />
 {:else}
   <Lobby />
 {/if}