⚡ Speed up client test CI

[baealex]

🎯 Goal

Speed up PR test feedback by removing unnecessary client test serialization and reducing avoidable frontend test setup cost.

🛠️ Core Changes

• Remove the client test:ci --maxWorkers 1 restriction so independent Vitest files can run in parallel.
• Move pure client unit specs to the Node test environment where DOM APIs are not required.
• Guard shared client test setup so it can run under both jsdom and Node environments.
• Document the release: no-impact label for CI, demo-only, development-environment, and other non-release-affecting changes.
• Replace the real debounce wait in SidebarSearch with fake timers.
• Simplify Note.external-change value changes where the protected contract is save/conflict behavior, not per-key typing behavior.

🧠 Key Decisions

• Kept user-event based interaction tests where keyboard/click behavior itself is the contract.
• Kept StrictMode coverage for the note external-change page test.
• Left larger page-test restructuring for a separate change because the current PR already reduces CI time with low-risk test-only changes.
• Classified this PR as release: no-impact because it changes CI/test operations and process docs without changing released runtime behavior.

🧪 Verification Guide

• pnpm test:ci should pass.
• pnpm --filter @ocean-brain/client lint should pass.
• pnpm check:encoding should pass.
• Local timing improved from about 97.36s to about 21.82s for pnpm test:ci on the same machine.

✅ Checklist

• Release impact label applied: release: no-impact
• Local validation passed: pnpm test:ci
• Local validation passed: pnpm --filter @ocean-brain/client lint
• Local validation passed: pnpm check:encoding
• PR title follows <emoji> <subject> with an English verb
• Release impact: release: no-impact