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/templates/pages/dictionary_preview.gohtml

@@ -0,0 +1,27 @@
+{{define "content" -}}
+<h1>Dictionary update — preview</h1>
+{{with .Data}}
+<section class="panel">
+<p>Comparing the uploaded archive against the active version <span class="pill">{{.ActiveVersion}}</span>. Review the changes below, then confirm to install and activate.</p>
+<form class="form" method="post" action="/_gm/dictionary/install">
+<input type="hidden" name="token" value="{{.Token}}">
+<label>Version <input type="text" name="version" value="{{.Version}}" required></label>
+<div><button type="submit">Update dictionaries</button></div>
+</form>
+</section>
+{{range .Variants}}
+<section class="panel">
+<h2>{{.Variant}}</h2>
+<p><strong>{{.AddedCount}}</strong> added, <strong>{{.RemovedCount}}</strong> removed.
+{{if .LargeRemoval}}<span class="warn">Large removal — double-check this is expected before updating.</span>{{end}}</p>
+<h3>Added{{if .AddedTruncated}} (showing first {{len .AddedSample}}){{end}}</h3>
+<p>{{range .AddedSample}}<code>{{.}}</code> {{else}}<span class="note">none</span>{{end}}</p>
+{{if .AddedTruncated}}<p class="note">… and more; the full list is not shown.</p>{{end}}
+<h3>Removed{{if .RemovedTruncated}} (showing first {{len .RemovedSample}}){{end}}</h3>
+<p>{{range .RemovedSample}}<code>{{.}}</code> {{else}}<span class="note">none</span>{{end}}</p>
+{{if .RemovedTruncated}}<p class="note">… and more; the full list is not shown.</p>{{end}}
+</section>
+{{end}}
+<p><a href="/_gm/dictionary">Cancel</a></p>
+{{end}}
+{{- end}}