feat(admin): online dictionary update — upload archive, preview word diff, install & activate

Replace the dictionary hot-reload with an online update flow in the GM console
(/_gm/dictionary): the operator uploads scrabble-dawg-vX.Y.Z.tar.gz, previews the
per-variant words added/removed against the active dictionary, and confirms to
install + activate it. Versions are immutable; in-progress games keep their pinned
version while new games use the new one.

- engine: DiffWords (enumerate both DAWGs, decode only the differences), OpenFinder,
  Registry.Finder, DictFiles; OpenWithVersions skips the .staging area.
- dictadmin: hardened release-archive validation + extraction (path-traversal,
  symlink, oversize, entry-count rejection) and staging -> install (atomic rename).
- game: active dictionary version persisted in the dictionary_state singleton
  (single source of truth, restored on boot), concurrency-safe accessor.
- storage: BACKEND_DICT_DIR is a named volume seeded from the image (nonroot-owned),
  so uploaded versions persist across redeploys; the build's DICT_VERSION labels the
  seed and equals the resident tag (BACKEND_DICT_VERSION).
- docs: ARCHITECTURE §5, FUNCTIONAL (+ru), backend README, TESTING, PRERELEASE.

Tests: engine + dictadmin unit; integration upload->preview->install->activate->
restart->pin->immutability->CSRF.

backend/internal/adminconsole/render_test.go

@@ -36,7 +36,8 @@ func TestRendererRendersEveryPage(t *testing.T) {
 		{"complaints", ComplaintsView{Items: []ComplaintRow{{ID: "c1", Word: "qi", Status: "open"}}, Status: "open", Pager: NewPager(1, 50, 1)}, "qi"},
 		{"messages", MessagesView{Items: []MessageRow{{ID: "m1", SenderID: "a1", SenderName: "Kaya", Source: "telegram", Body: "good luck", GameID: "g1"}}, Pager: NewPager(1, 50, 1)}, "good luck"},
 		{"complaint_detail", ComplaintDetailView{ID: "c1", Word: "qi", Variant: "scrabble_en"}, "Resolve"},
-		{"dictionary", DictionaryView{Variants: []VariantVersions{{Variant: "scrabble_en", Latest: "v1", Versions: []string{"v1"}}}, Changes: []DictChangeRow{{Variant: "scrabble_en", Word: "qi", Action: "add"}}}, "Hot-reload"},
+		{"dictionary", DictionaryView{ActiveVersion: "v1.0.0", Variants: []VariantVersions{{Variant: "scrabble_en", Versions: []string{"v1.0.0"}}}, Changes: []DictChangeRow{{Variant: "scrabble_en", Word: "qi", Action: "add"}}}, "Update dictionaries"},
+		{"dictionary_preview", DictionaryPreviewView{Version: "v1.1.0", Token: "0123456789abcdef0123456789abcdef", ActiveVersion: "v1.0.0", Variants: []VariantDiffRow{{Variant: "scrabble_en", AddedCount: 2, RemovedCount: 1, AddedSample: []string{"qi", "za"}, RemovedSample: []string{"xqz"}, RemovedTruncated: true}}}, "v1.1.0"},
 		{"broadcast", BroadcastView{ConnectorEnabled: true}, "Post to the game channel"},
 		{"message", MessageView{Heading: "Done", Body: "ok", Back: "/_gm/"}, "Done"},
 	}