fix(ai-chat): plural token strings + cover reasoning UI + cleanups (#151 review)

Review of #158 (Request changes) — core logic verified correct; addressed the
test-coverage + localization items:

1. i18n pluralization: the token-count keys were called with {count} but had one
   form, so ru-RU always rendered the genitive ("1 токенов"). Added _one/_other
   (en) and _one/_few/_many (ru: токен/токена/токенов) for both "Thinking… ·
   {{count}} tokens" and "Thinking · {{count}} tokens"; de-duped the PR-added
   duplicate "Thinking" key. Call sites unchanged.
2. ReasoningBlock: new reasoning-block.test.tsx (4 branches: authoritative count
   wins / estimate fallback / header-only when count-but-no-text / body render).
3. Reasoning-token attribution: extracted the #151 anti-double-count rule into a
   pure `reasoningTokensForPart(message)` (single reasoning part -> authoritative
   turn total; multiple/none -> undefined so each estimates). message-item uses
   it; removed the now-dead lastReasoningIndex reduce (review #5). Unit-tested.
6. adopt-chat-id.ts: refreshed 3 stale `chatStreamStartMetadata` ->
   `chatStreamMetadata` comment references.
7. chat-markdown.test.ts: assert the export footer's `reasoning: N` line appears
   when reasoningTokens>0 and is absent at 0/undefined.

Skipped optional #4 (mantine useThrottledCallback): the manual throttle has two
distinct exit paths (turn-end revert-to-null + the captured-total trailing emit)
with no guarding test; remapping risks the streaming behavior — non-blocking.

Client tsc clean; ai-chat suite green (171 tests).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/public/locales/ru-RU/translation.json

@@ -681,8 +681,13 @@
   "{{name}} is typing…": "{{name}} печатает…",
   "Thinking…": "Думаю…",
   "Thinking… · {{count}} tokens": "Думаю… · {{count}} токенов",
-  "Thinking": "Размышления",
+  "Thinking… · {{count}} tokens_one": "Думаю… · {{count}} токен",
+  "Thinking… · {{count}} tokens_few": "Думаю… · {{count}} токена",
+  "Thinking… · {{count}} tokens_many": "Думаю… · {{count}} токенов",
   "Thinking · {{count}} tokens": "Размышления · {{count}} токенов",
+  "Thinking · {{count}} tokens_one": "Размышления · {{count}} токен",
+  "Thinking · {{count}} tokens_few": "Размышления · {{count}} токена",
+  "Thinking · {{count}} tokens_many": "Размышления · {{count}} токенов",
   "Agent role": "Роль агента",
   "AI chat": "AI-чат",
   "AI chat is disabled for this workspace.": "AI-чат отключён для этого рабочего пространства.",