Phase 28 (Step 10): docs — diplomail UI topic + FUNCTIONAL mirror

- `ui/docs/diplomail-ui.md`: new topic doc covering the wire
  surface, recipient-by-race-name decision, threading model,
  translation toggle, push events, badge, layout, and
  accessibility.
- `docs/FUNCTIONAL.md` §11.4 grows a paragraph that records the
  UI's per-race threading rule, the absent read-receipt UX, and
  the recipient-by-race-name compose path. Mirrored verbatim into
  `docs/FUNCTIONAL_ru.md`.
- `ui/PLAN.md` Phase 28 marked done with a "Decisions during
  stage" block matching the implementation plan, and the artifact
  list updated to the actual file set.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/PLAN.md

@@ -3070,9 +3070,73 @@ bottom):
 - animated transitions when survivors re-distribute after an
   elimination (currently hard-jumps).
 
-## Phase 28. Diplomatic Mail View
+## ~~Phase 28. Diplomatic Mail View~~
 
-Status: pending.
+Status: done (pending CI gate).
+
+Decisions baked in during implementation:
+
+1. **Transport: ConnectRPC `user.games.mail.*`.** Eight new
+   authenticated commands (inbox / sent / message.get / send /
+   broadcast / admin / read / delete) plumbed end-to-end through
+   the existing gateway → backend REST surface. Schemas in
+   `pkg/schema/fbs/diplomail.fbs`; constants in
+   `pkg/model/diplomail/diplomail.go`; gateway translation in
+   `gateway/internal/backendclient/mail_commands.go`.
+2. **Recipient by race name.** The send / admin endpoints accept
+   an alternative `recipient_race_name` field; backend resolves it
+   via `Memberships.ListMembers(gameID, "active")`. The UI feeds
+   the picker straight off `report.races[].name` — no client-side
+   memberships RPC.
+3. **`sender_race_name` snapshot.** New nullable column on
+   `diplomail_messages`, populated for `sender_kind='player'`
+   senders that have an active membership at send time. Drives the
+   per-race threading on the client.
+4. **/sent returns full message detail.** Backend's bulk sent
+   listing now returns the same `UserMailMessageDetail` shape as
+   `/inbox`, one row per (message, recipient). The UI collapses
+   broadcasts by `message_id` into a single stand-alone item.
+5. **Threading + stand-alones.** `MailStore.entries` groups
+   personal messages by the other party's race name. System,
+   admin, and outgoing broadcasts render as stand-alone items in
+   the same list pane.
+6. **No read receipts.** `read_at` and `deleted_at` drive the
+   badge counter and soft-delete affordance but are never shown
+   to the user.
+7. **Header badge.** Inline pill on the view-menu "diplomatic
+   mail" row, fed by `mailStore.unreadCount`. No always-visible
+   chrome added.
+8. **Push event reuse.** A new
+   `eventStream.on("diplomail.message.received", …)` handler in
+   `routes/games/[id]/+layout.svelte` parses the verified payload,
+   refreshes the inbox, and raises a `toast.show` with a "view"
+   deep-link.
+9. **Translation toggle.** Per-message Show original / Show
+   translation toggle inside both `thread-pane.svelte` and
+   `system-item-pane.svelte`; the body defaults to the cached
+   translation when present.
+
+Artifacts (delivered):
+
+- backend: `internal/postgres/migrations/00001_init.sql`,
+  `internal/diplomail/{types.go,store.go,service.go,admin_send.go,diplomail_e2e_test.go,README.md}`,
+  `internal/server/{handlers_user_mail.go,handlers_admin_diplomail.go}`,
+  `openapi.yaml`;
+- wire: `pkg/schema/fbs/diplomail.fbs` + generated Go and TS
+  bindings; `pkg/model/diplomail/diplomail.go`;
+- gateway: `gateway/internal/backendclient/{mail_commands.go,routes.go,mail_commands_test.go}`,
+  `gateway/cmd/gateway/main.go`;
+- ui: `ui/frontend/src/api/diplomail.ts`,
+  `ui/frontend/src/lib/mail-store.svelte.ts`,
+  `ui/frontend/src/lib/active-view/mail.svelte` (+ subdir
+  `mail/{thread-list,thread-pane,system-item-pane,compose,system-titles}.svelte|.ts`),
+  `ui/frontend/src/lib/header/view-menu.svelte`,
+  `ui/frontend/src/routes/games/[id]/+layout.svelte`,
+  `ui/frontend/src/lib/i18n/locales/{en,ru}.ts`;
+- docs: `ui/docs/diplomail-ui.md`, `docs/FUNCTIONAL.md` §11.4 +
+  mirror in `docs/FUNCTIONAL_ru.md`.
+
+Original phase brief follows.
 
 Goal: implement a mail inbox and compose flow as a dedicated view that
 replaces the map.