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.

docs/TESTING.md

@@ -15,7 +15,12 @@ tests or touching CI.
   `go test -tags=integration -count=1 -p=1 ./backend/...` (needs Docker), guarded
   by a separate CI workflow (`integration.yaml`; Ryuk disabled, serial). Slow.
 - **UI** *(introduced with the UI in Stage 7)* — Vitest (unit) + Playwright
-  (e2e), mirroring the chosen plain-Svelte + Vite toolchain.
+  (e2e), mirroring the chosen plain-Svelte + Vite toolchain. Stage 8 adds Vitest for
+  the new FlatBuffers codecs (friend list, invitation, stats), the win-rate
+  derivation and the GCG share/download choice, plus Playwright specs against the
+  mock for the friends screen (code issue/redeem, accept a request), the lobby
+  invitations section, the stats screen, profile editing, and the GCG export's
+  finished-only visibility.
 - **Engine** *(Stage 2+)* — correctness of scoring and move generation is owned
   by `scrabble-solver`'s own GCG-backed tests. `backend/internal/engine` adds, on
   top of the embedded solver: per-variant smoke tests (load all three committed
@@ -48,7 +53,11 @@ tests or touching CI.
   content and block-visibility rules, the nudge turn/rate-limit rules, the
   invitation flow (all-accept starts the game, decline cancels, lazy expiry,
   inviter-only cancel), and the email confirm-code flow (request/confirm, taken
-  email, expiry and attempt-cap) with a fixture mailer.
+  email, expiry and attempt-cap) with a fixture mailer. Stage 8 adds the
+  **befriend-an-opponent** gate (a request needs a shared game), the **permanent
+  decline** and 30-day re-send rule, the **one-time friend code** (issue/redeem,
+  self/single-use, decline-bypass), `ListInvitations`, the zero-value `GetStats`, and
+  the GCG **finished-only** gate.
 - **Robot** *(Stage 5+)* — `backend/internal/robot` unit-tests the pure strategy:
   the ≈ 40% play-to-win split over many seeds, the right-skewed move-delay
   (bounds, ~10-min median, determinism), the margin selection (win/lose, in-band
@@ -70,7 +79,9 @@ tests or touching CI.
   (guest auth, unauthenticated rejection, unknown message type). The backend gains
   the **guest** lifecycle (a guest plays an auto-match to a natural end yet accrues
   no statistics) and the **email-as-login** flow (request/verify, returning user)
-  in `inttest`.
+  in `inttest`. Stage 8 adds gateway transcode round-trips for the new social/account
+  operations (friends list, friend code issue/redeem, invitation create, stats, GCG,
+  the profile-update away round-trip) and a `notify`-event constructor round-trip.
 
 ## Principles