feat(admin): manual account blocking (suspensions)

Operator-driven hard block, the counterpart to the soft high-rate flag: permanent or until a date, with an optional reason chosen from an editable en+ru picklist (snapshotted onto the block). A block forfeits the player's active games (opponent wins, as a resignation) and cancels their open matchmaking games. A backend gate refuses a blocked account on every /api/v1/user/* route except the block-status probe with 403 account_blocked, which threads through the gateway as the Execute result_code; the UI surfaces it as a terminal blocked screen and stops all push/poll. Temporary blocks self-expire; the operator can unblock at any time (lost games stay lost). Sessions are not revoked, so the blocked client can still reach the exempt block-status endpoint.

Backend: migration 00003 (account_suspensions + suspension_reasons) + jet regen; account suspension store; game.ForfeitAllForAccount; requireNotSuspended gate + block-status endpoint; admin console block/unblock + Reasons CRUD. Wire: fbs BlockStatus + account.block_status gateway op. UI: blocked screen, app state, transport/codec, i18n. Docs: ARCHITECTURE, FUNCTIONAL(+ru), PRERELEASE (AB).

backend/internal/adminconsole/templates/layout.gohtml

@@ -18,6 +18,7 @@
     <a href="/_gm/complaints"{{if eq .ActiveNav "complaints"}} class="active"{{end}}>Complaints</a>
     <a href="/_gm/messages"{{if eq .ActiveNav "messages"}} class="active"{{end}}>Messages</a>
     <a href="/_gm/throttled"{{if eq .ActiveNav "throttled"}} class="active"{{end}}>Throttled</a>
+    <a href="/_gm/reasons"{{if eq .ActiveNav "reasons"}} class="active"{{end}}>Reasons</a>
     <a href="/_gm/dictionary"{{if eq .ActiveNav "dictionary"}} class="active"{{end}}>Dictionary</a>
     <a href="/_gm/broadcast"{{if eq .ActiveNav "broadcast"}} class="active"{{end}}>Broadcast</a>
     <a href="/_gm/grafana/">Grafana ↗</a>