fix(chat): include cache-read tokens in context usage

[qiontoo]

Summary

• include cacheReadTokens in the chat input context usage display
• carry cache-read/cache-write usage through session summaries and live usage updates
• add a regression test covering cached-token context display

Closes #838

Test Plan

• npx vitest run tests/client/chat-input-token-cache.test.ts --reporter=dot
• npx vue-tsc -b
• npx tsc --noEmit -p packages/server/tsconfig.json
• git diff --check